Allah, the Wise, has said:
يَا أَيُّهاَ النَّاسُ کُلُوا مِمَّا فِي الأَرْضِ حَلاَلاً طَيِّباً
"O mankind! eat the lawful and good things out of what is in the earth." 
Imam Kadhim (as) said:
إِنَّ الْحَراَمَ لاَ يُنْمِي وَ إِِنْ نُمِيَ لَمْ يُباَرَكْ فِيهِ.
"Surely, the unlawful things do not grow (and multiply) and if they ever do, they are never blessed." 
Consumption of lawful things results in soundness of health and a good Hereafter, whereas consumption of unlawful things causes hardening of the heart - the gravest of diseases for the heart. Its ill effects are also seen to manifest in one's progeny and even becomes cause for a person to stand up in opposition to Allah! The Prophets and the auliya never ate unlawful things and always counseled their ummah to earn a lawful income and desist from unlawful things.
Why should one who shall eventually be in need of only a grave and a few meters of shroud strive to collect wealth by unlawful means only to leave it for the others, not to mention its burden and responsibility?
In regards to earning a lawful income, it has been reported from the Noble Prophet (sa) that worship consists of seventy parts, the most excellent of them being the earning of a lawful income. The act of earning a lawful income causes a person's heart to become illuminated, his acts of worship to be accepted, and the person finds himself in the continuous protection of Allah.
1 – The Jews and the Unlawful Food
When the Noble Prophet (sa) was seven years of age, the Jews (having perceived signs of Prophethood in him and therefore deciding to test him) discussed amongst themselves: "We have read in our Scriptures that the Prophet will distance himself from unlawful and dubious food, so let us test him."
Thus, they stole a fowl and gifted it to Abu ±alib so that the members of his family could eat it. All of them ate, except for the Noble Prophet (sa) who did not touch the food. When questioned, he replied: "This fowl is unlawful and Allah has protected me from unlawful things."
After this incident, the Jews got hold of a neighbour's fowl with the intention of paying him later and sent it to Abu ±alib, but once again the Noble Prophet (sa) refused to eat the food, saying: "This food is dubious (with respect to its lawfulness)."
When the Jews came to know of these incidents, they commented, "This child shall come to possess a lofty rank and status."
2 – By Unlawful Means
During the period when Imam Baqir (as) was in the captivity of Mansur Dawaniqi (the second Abbasid Caliph), he used to eat very little food. Once, a righteous lady who was the follower of the Ahlul Bayt prepared two loaves of bread by lawful means and sent it to the Imam (as) so that he could eat them. The jail warden said to Imam (as): "A certain pious lady, who happens to be your follower has sent these loaves as a present for you and has sworn that it has been made out of lawful means and has requested you to eat them."
The Imam (as) refused to accept the loaves and asked for them to be returned to the lady.
"Tell her: We know that your food is lawful; however, since you have made it reach us by unlawful means, it does not befit us to eat it," he said.
3 – The Satan's Snare
One of the students of ھyatullah Sheikh Murtaza Ansari recounts: "One night, while we were in Najaf engaged in studies under the tutelage of the Sheikh, I saw Shaitan in my dreams. He was armed with numerous ropes and cords in his hands.
"What are these ropes for?" I asked him.
He replied, "I put them around people's necks, draw them towards myself and ensnare them. Just last night, I had put one of these strong cords around the Sheikh's neck and had managed to drag him from his room to the middle of the street in which his house is situated, but unfortunately he escaped from my clutches and returned home."
The next morning when I went to the Sheikh, I related the previous night's dream to him.
"Shaitan has spoken the truth," the Sheikh explained. "That accursed had desired to beguile me but by the grace of Allah, I escaped from his grasp. Yesterday, I did not have any money to purchase something for the house. I said to myself: 'I have one riyal from the money of Imam al-Zaman (as) and there is still some time before I can put it to use. I shall borrow it for now and repay it later.'
"I left the house with that money, but as I was about to purchase the item which I needed, I said to myself: "How do I know I shall be able to repay this debt later?" I wavered and then all of a sudden decided against going ahead with the purchase. As soon as I returned home, I put the money back in its place."
4 – The Caliph's Food
Once, in a gathering of Harun Rashid (the fifth Abbasid Caliph), which included a number of aristocrats, the conversation drifted towards Buhul and his insanity.
When it was time for lunch, a king's luncheon was spread out, and a delicacy especially prepared for Harun was placed before him. Harun handed this food to one of his slaves and ordered him to take the food to Buhul, in the hope of drawing him towards himself with this benevolent act.
When the slave brought the food before Buhul, who was seated in the ruins of a broken down house, he noticed that some dogs nearby were tearing apart and eating the carcass of a dead donkey. Buhul refused to accept the food.
"Place the food before the dogs," he said to the slave.
"This is the Caliph's special food and he has sent it to you out of his respect for you. Do not insult the Caliph!" ordered the slave.
Buhul responded, "Lower your voice for if the dogs come to know of this, even they would refuse to eat this food." (Since, with regards to the riches at the Caliph's disposal, it is not known which part of it is lawful and which part is not.)
5 – Aqil
Once, Aqil, the brother of Imam Ali (as), seeking some monetary help, asked the Imam (as) to give him something because he was poor.
The Imam (as) said, "Be patient till I distribute the money amongst the other Muslims for then, I shall give you your share too."
But when Aqil persisted with his request, the Imam (as) said to a person: "Take Aqil by the hand towards the market and ask him to force open the lock of one of the shops and take away everything from it!"
Aqil immediately asked, "You want me to be arrested as a thief?"
"And by giving you money from the public treasury of the Muslims you want me to be looked upon as a thief?" the Imam (as) retorted.
"I shall go to Muaawiyah," replied Aqil.
Imam Ali (as) suggested to him to do as he pleased. Aqil went to Muaawiyah to seek help from him whereupon Muaawiyah gave him a hundred thousand dirhams and said: "Ascend the pulpit and inform the people as to how Ali (as) behaved with you and how I cooperated with you."
Aqil climbed up the pulpit and, after thanking and praising Allah, said: "O' People! When I sought from Ali (as) his religion, he abandoned me - his brother, and adhered to his religion. However, when I approached Muaawiyah, he gave me preference over his religion."
 Noble Qur'an, Suratul Baqarah (2), Verse 168
 Jamea al-Saaadat, Volume 2, Page 167
 Safinatul Bihar, Volume 1, Page 297
 Dars-hai Az Zindagi-e-Payambar, Page 31; Biharul Anwar, Volume 15, Page 336
 Lataif al-Tawaif, Page 44
 Sima-e-Farzanegan, Page 430; Zindagani Wa Shakhsiyyat-e-Sheikh Ansari, Page 88
 Hikayat-ha-e-Shanidani, Volume 1, Page 120
 Pand-e-Tarikh, Volume 1, Page 180; Al-Sawaiqul Muhriqah