Allah, the Wise, has said:
إِنَّ اللٌّهَ لاَ يُحِبُّ مَنْ کَانَ خَوَّاناً أَثِيْماً
"Surely Allah does not love him who is treacherous, sinful." 
Imam Sadiq (as) said:
لَيْسَ لَكَ أَنْ تَأْتَمِنَ الْخَائِنُ
"It is not for you to trust a treacherous person." 
If a thing such as money, a business, a car or the like is placed as trust in one's possession, one should not be unfaithful to it, spoil or disfigure it or deny having received it as a trust.
A person with this vice does not have credibility neither in the eyes of Allah nor in the eyes of the people. He drops down from the level of faith and the (evil) reaction of his deed rebounds - affecting him, his wealth and his family in a detrimental way.
It has been strongly advised that one should not be deceived by a person's (prolonged) prayers and (numerous) fasts - for it is possible that the person may have simply become fond of performing these acts - instead, one should test a person for truthfulness, and faithfulness with respect to the trusts (placed in his custody).
One should never place a trust in the possession of an unfaithful person. Lending money or giving one's daughter in marriage to a treacherous person is disapproved and if one does so and then happens to suffer loss or harm, it is only himself that he should censure and rebuke.
1 – The Treacherous Minister
During his reign, Gushtasp had a minister by the name of Rast Rawishan. As a result of this prestigious name, Gushtasp held him in high esteem and favored him over the other ministers.
This minister exhorted Gushtasp into oppressing the subjects and confiscating their property in the belief that the orderliness of the kingdom's affairs was dependent upon the treasury and that the subjects ought to be poor in order that they remained subservient and obedient. He himself had not only accumulated a lot of wealth but had come to harbour animosity towards Gushtasp.
One day, when Gushtasp came to the treasury, he realised that there was no money to pay his workers. Furthermore, his cities were falling into ruin and the people were in distress. This left Gushtasp in a state of bewilderment.
Out of sheer despair, he climbed onto his horse and set off into the wilderness. As he wandered, his eyes fell upon a flock of sheep in the distance. When he came nearer, he observed that the sheep were sleeping while a dog lay suspended from the gallows. In astonishment, he asked the shepherd the reason for killing the dog. The shepherd replied: "This dog was a loyal animal; I nurtured him and trusted him to protect the sheep. After a period, he came in contact with a she-wolf and both became friends. When night fell, the she-wolf would take hold of a sheep, eat half of it, and leave the other half for the dog.
"One day, I noticed a decrease in the number of sheep and after investigation, came to know of the dog's treachery. Therefore, I hung him up on the gallows so that it is known that the consequence of treachery and evil is torture and punishment!"
Hearing this, Gushtasp thought to himself, "My subjects are like these sheep and I am like the shepherd; I must investigate and study the condition of the people so that I know the cause for their deplorable state."
He returned to his court and asked for the list of prisoners who were locked in his prisons. Studying the list, he concluded that his minister, Rast Rawishan, had imprisoned all of them and that he was the cause of all the evils and troubles. He had the minister hanged and admitted to himself that he had been deceived by his name.
Gradually he made the kingdom prosperous, rectified the past damages, paid attention to the state of the captives and refrained from trusting anyone, ever again.
2 – Treachery during Ziarat
Al-Hajj Hasan, the son of ھyatullah al-Hajj Husain ±abataba`i Qummi, relates: "I had come to Tehran from Mashhad for the treatment of my eyes. During that time, one of the traders of Tehran who was known to me, had travelled to Khurasan for the ziarat of Imam Riza (as).
One night, I dreamt that I was in the sanctuary of Imam Riza (as) who was seated on the sepulcher. Suddenly, I observed that the trader shot an arrow at the Imam (as) which greatly troubled him. For the second time, from another direction of the sepulcher, he let fly another arrow towards the Imam (as) and again the Imam (as) was deeply troubled. The third time, the trader shot an arrow from behind but on this occasion, the Imam (as) collapsed on his back. Shocked and petrified, I woke up from my sleep.
When my eye treatment was completed, I wished to go back to Khorasan but then decided to stay on till the trader returned from Khorasan. When he returned, I spoke to him and asked him certain questions but could not get to the bottom of the matter. Eventually, I narrated my dream to him, whereupon, with tears flowing from his eyes, he explained: "One day, having entered the sanctuary of Imam Riza (as), I noticed that before me stood a lady with her hand placed on the sepulcher. I placed my hand on hers and so the lady went to the other side of the sepulcher. I followed her there and once again placed my hand on hers. This time, the lady went behind the sepulcher. When she had placed her hand upon it, I did exactly the same as before and asked her where she was from. She replied that she was from Tehran; we became friends and returned to Tehran together."
3 – The Daughter's Disloyalty towards the Father
Satrun, whose title was Dhizan was the king of Hadhar, a state located between the rivers Tigris and Euphrates. In Hadhar, there was a beautiful palace by the name of Jausaq.
Once, the king attacked a city that was under the control of Shapur; looting and plundering it, he massacred a great number of its people and eventually occupied it. In the process he managed to take Shapur's sister as captive.
When Shapur came to know of this, he gathered his army and set out towards the king. Dhizan had locked himself inside a strong fort which Shapur laid siege to. This siege continued for a period of four years but Shapur was unable to penetrate the fort.
One day, Dhizan's daughter, Nadhirah, an extremely beautiful maiden, was strolling outside the fort when Shapur's eyes fell on her. He was instantly captivated by her beauty. He sent word to her that if she helped him conquer the fort, he would marry her. During one of the nights, Nadhirah, who too had fallen for Shapur, got the guards of the fort intoxicated and opened its doors to Shapur's forces. In the battle that ensued, her father Dhizan was eventually killed.
In keeping with his promise, Shapur married Nadhira. One night he noticed that there was blood on her bed. As he set about to investigate the cause of it, he observed that a strand of thick hair lay on her bed and this had caused her delicate and subtle body to become scratched and injured.
"What did your father feed you with?" he asked her.
"Yolk of eggs, brain of lambs, butter and honey," she replied.
When he heard this, Shapur reflected for a while and then said:
"Despite these comforts provided by your father, you were not loyal to him; how can you ever be loyal and faithful to me?"
He ordered that she be tied to a horse's tail and the animal made to gallop through the desert so that the desert thorns become coloured by the blood of this treacherous and unfaithful daughter.
4 – The Indian and the Sixth Imam
Imam Kadhim (as) narrates: "One day, I was with my father in the house when a friend entered and told my father that some people were standing outside, waiting to see him. My father asked me to find out who they were. As I went outside, I saw numerous camels laden with chests and a man seated on a horse.
"Who are you?" I asked the man.
"I am from India and seek the honour of meeting the Imam (as)," he replied.
I returned to my father and informed him about the man outside.
"Don't let this impure and treacherous person enter the house," he instructed, and so I did as I was told. The people pitched their tents at that very place near the house and waited for a long time till Yazid Ibn Sulaiman and Muhammad Ibn Sulaiman intervened and procured permission for them to meet my father.
When the Indian entered, he sat down before the Imam (as) with folded knees and said: "May you have good health! I am from India and the king has sent me with some presents which are to be handed over to you. For several days I have been seeking permission to enter but you have been refusing to meet me. Do the Prophets' children conduct themselves in this manner?"
My father (as) lowered his head and answered, "You will come to know the reason for it later." He then asked me to open the letter that the Indian had brought. In the letter, the king of India had extended his greetings. Then he had written: "It is because of you that I have been guided aright. I had been presented with an extremely beautiful slave-girl, and I found none, save you who could be worthy of possessing her and so, in addition to some clothes, ornaments and perfumes, I gift her to you. Out of one thousand persons, I selected one hundred, and from them, I selected ten and from the ten, I have short listed one person, Mizan Ibn Khabbab, who is trustworthy. I send him to you together with the slave-girl and the presents."
My father turned to the Indian and said, "O' Unfaithful person! Turn back, for I shall never accept a trust that has been subjected to unfaithfulness."
The Indian swore that he had not been unfaithful, however my father told him, "If your clothes were to testify that you had been unfaithful with respect to the slave-girl, would you become a Muslim?"
"Do forgive me!" implored Mizan.
"Then write your deeds to the king of India."
"If you know something in connection with the issue, you write it down," said Mizan.
The man had a sheepskin over his shoulder; the Imam (as) told him to place it on the ground.
My father then offered a two-rak'at prayer after which he went into prostration and supplicated:
أَللٌّهُمَّ إِنِّي أَسْـئَـلُكَ بِمَعَاقِدِ الْعِزِّ… إِيْماَناً مَعَ إِيْماَنِهِمْ
then raising his head, he turned towards the sheepskin and said:
"Speak out all that you know about this Indian."
The sheepskin began to speak as if it were a sheep, and said:
"O' Son of the Prophet of Allah! The king considered this person to be trustworthy and had laid great emphasis with regards to protecting the slave-girl and the gifts. When we had travelled some distance, we reached a desert where heavy rains lashed us. All our belongings had become soaked due to the rain. A short while later the clouds cleared away and the sun began to shine. At that point, this unfaithful person called out to the servant, who had been accompanying the slave-girl, and dispatched him towards the city to purchase something. When the servant had gone, he said to the slave-girl: "Enter into this tent which we have pitched beneath the sun so that your clothes and body dry up. The slave-girl entered the tent and pulled up her clothes up to her calves. As soon as his eyes fell upon her legs, he became mesmerized and persuaded the slave-girl into being unfaithful."
The Indian, disturbed and distressed at witnessing the sheepskin (speaking out his misdeed), confessed to his misdemeanour and sought forgiveness. The sheepskin returned to its original state and the Imam (as) ordered him to put it on. As soon as the Indian had placed it over his shoulder, it encircled and tightened itself around his neck and the man was almost on the verge of being strangulated when the Imam (as) said: "O' Sheepskin! Leave him so that he can return to the king who would be the most appropriate person to punish this man for his unfaithfulness."
The sheepskin reverted back to its original state. The Indian, overcome with fright, implored the Imam (as) to accept the gifts.
"If you become a Muslim, I shall gift the slave-girl to you," said the Imam (as).
But he declined the offer. The Imam (as) then accepted the presents, but refused to take the slave-girl, and the man returned to India.
After a month, a letter arrived from the king of India, in which, after extending his greetings he wrote: "You accepted that which did not have any significant value, whereas rejected that which was valuable. This left me greatly disturbed and I said to myself: 'The children of the Prophets possess divine insight and wisdom and it is possible that the person who had escorted the slave-girl, might have exhibited unfaithfulness.' And so, I wrote a letter in your name to myself, and said to the man that your letter had reached me in which you had mentioned his unfaithfulness. I said to him: 'Nothing, except the truth, can save you', whereupon he confessed and related to me the entire episode of his unfaithfulness with regards to the slave-girl and the incident of the sheepskin. The slave-girl also confessed and so I ordered both of them to be beheaded.
"I bear witness to the Unity of Allah and the Prophethood of the Noble Prophet (sa) and have to state that I shall personally arrive in your presence later."
Before long, he arrived in Madinah after having abdicated his kingship and transformed himself into a true Muslim.
 Noble Qur'an, Suratul Nisa (4), Verse 107
 Biharul Anwar, Volume 78, Page 248
 Literally, his name means ‘a truthful worker'.
 Jawame' al-Hikayat, Page 313; Siyasatnameh-e-Khwajah Nidham al-Mulk
 Rahnama-e-Saaadat, Volume 1, Page 257; Jamea al-Durar, Volume 1, Page 448
 Namunah-e-Maaarif, Volume 5, Page 142, Al-Mustatraf, Volume 1, Page 210
 Pand-e-Tarikh, Volume 1, Page 217; Biharul Anwar, Volume 11, Page 136