Allah, the Wise, has said:
فَاعْبُدِ اللهَ مُخْلِصاً لَهُ الدِّيْن
"So worship God (alone), being sincere to Him in religion" 
Imam A'li (a.s.) said:
أخْلِصِ الْعَمَلَ يُجْزِكَ مِنْهُ الْقَلِيْلُ
"Perform your deeds with sincerity, for (then, even) a slight of it shall be sufficient for you." 
Sincerity is the key to the acceptance of all deeds. A person whose deeds are accepted by Allah, however trivial they may be, is a sincere person and one whose deeds, in spite of being plentiful are rejected by Allah, is not of the sincere ones.
A sincere person strives to purify his soul from vices and exerts himself to perform (good) deeds and maintain (sincerity of) intention so that Allah accepts his deeds.
The level of intention, knowledge and deeds is related to spiritual purification and refinement, and if a sincere person were to pay due attention to his inner self, he would come to perceive the true concept of Unity of God. The lowest degree of sincerity is when a person exerts himself to the best of his ability, neither anticipating rewards for his deeds nor attaching any importance to them. 
1 - Three Persons in a Cave
The Holy Prophet (s.a.w.) has related:
"Three persons from the tribe of Bani Israel got together and started out on a journey. On the way, clouds gathered above them and it started to pour with rain and so they sought shelter in a nearby cave.
Suddenly, a large boulder slipped and blocked the entrance to the cave, trapping the three inside and transforming the day into a dark night for them. They had no other alternative except to turn to God for help.
"Let us use our sincere deeds as a means to obtain deliverance from this predicament," suggested one of them.
All the others agreed with the suggestion.
One of them said, "O’ Lord! You are aware that I have an extremely attractive cousin and that I was infatuated and obsessed with her. One day, finding her alone, I took hold of her and wanted to satisfy my carnal desires, when she spoke out to me saying: O’ My cousin!Fear God and do not harm my chastity’. Hearing this, I crushed my lustful tendencies anddecided against the evil act. O’ Lord! If that deed of mine had been out of absolute sincerityand only for the purpose of acquiring Your pleasure, deliver us from grief and perdition." Suddenly they witnessed that the huge boulder had moved away a little, faintly brightening up the interior of the cave.
The second person spoke out,"O’ Lord! You know that I had a father and a mother, so old that their bodies had bent over due to their excessive age, and that I used to tend to them regularly. One night, having brought them their food, I observed that both of them were asleep. I passed the entire night near them, the food in hand, without waking them up for fear of disturbing them. O’ Lord! If this deed of mine had been only for the sake of Your pleasure and happiness, open up a way for us and grant us salvation."
As he completed his speech, the group noticed that the boulder had moved aside a little more.
The third person supplicated, "O’ Knower of every hidden and manifest! You know Yourself that I had a worker who used to work for me. When his term had reached its termination, I handed over to him his wages, but he was not pleased and desired more and, in a state of dissatisfaction and displeasure, he went away. I used his wage to purchase a goat, which I looked after separately and very soon I had a flock in my possession. After a period of time,the worker again approached me for his wage and I pointed towards the flock of sheep.Initially, he thought I was ridiculing him, but later, realizing my seriousness, took the entireflock and left. O’ Lord! If this act had been prompted by sincerity and had only been for Your pleasure, deliver us from this quandary."
At this point the entire boulder moved aside from the mouth of the cave and all three emerged from it, joyous and ecstatic, and continued their journey.
2 - A’li (a.s) on the Chest of A’mr
A'mr Ibn A’bd Wudd was a warrior who, in battle, was a match for a thousand soldiers. In the battle of Ahzaab, he challenged the Muslim soldiers to fight him but none possessed the courage to stand up before him till Imam A'li (a.s.) presented himself before the Holy Prophet (sa.w) and sought permission to fight him.
The Holy Prophet (s.a.w.) said to Ali, "Do realize that this is A'mr Ibn A’bd Wudd."
Imam A'li (a.s.) humbly stated, "(And) I am A'li Ibn Abi Talib,"and then proceeded towards the battle-field and stood before A'mr.
After a fierce encounter, Imam A'li (a.s.) eventually knocked him down and sat on his chest.
Witnessing this, the entire Muslim army importuned the Holy Prophet (s.a.w.): "O’ Messenger of Allah, order A'li (a.s.) to kill A'mr immediately."
"Leave him alone for he is more aware of his deeds than anyone else," replied the Holy Prophet (s.a.w.)
When A'li (a.s.) had severed the head of A'mr, he brought it to the Holy Prophet (s.a.w.), who questioned him (peace be upon him), "O’ A'li! What caused you to hesitate before killing A'mr?"
He said, "O’ Messenger of Allah! When I had floored him, he abused me, as a result of which I was overcome by rage. I feared that if I were to kill him in that state of anger, it would be for consoling myself and pacifying my soul. So I stepped away from him till my fury subsided when I returned to sever his head from his body only for the happiness of Allah and in obedience to Him."
It was because of this sincerity and invaluable combat on the part of Imam Ali that the Holy Prophet (s.a.w.) said:
"The strike of the sword of A'li on the day of the battle of Khandaq is superior to the worship of all men and jinn.",
3 - Shaitan and the Pious Worshipper
In the tribe of Bani Israel, there once lived a pious worshipper. One day, the people informed him that at a certain location, there existed a tree that was being worshipped by a tribe. When he heard this, the man flew into a rage, picked up his axe and set out to cut down the tree.
Iblees, appearing before him in the form of an old man, asked, "Where are you headed for?" He replied, "I intend to cut down the tree, which is being worshipped so that the people worship Allah instead." 
"Hold yourself till you hear what I have to say," said Iblees to him.
The worshipper urged him to carry on.
Iblees continued, "God has His own Prophets and if it had been essential to cutdown the tree, He would have sent them to perform the task."
However, the worshipper did not agree with Iblees and continued on his way.
"There is no way I shall let you do it," said Iblees angrily, and he began to wrestle with the man. In the ensuing contest, the pious worshipper hurled Iblees onto the ground.
"Wait! I have something else to say to you", pleaded Iblees. "Listen! You are a poor man. If you could possess some wealth by means of which you could give alms to the other worshippers it would be much better than cutting the tree. If you refrain from cutting the tree, I shall place two dinars beneath your pillow everyday."
The pious person said thoughtfully, "If you speak the truth, I shall give one dinar in charity while the other dinar I shall put to my use. This is better than cutting down the tree; in any case, I have neither been ordered to perform this task nor am I a Prophet to burden myself with unnecessarygrief and anxiety."
Thus, he acceded to the request of Shaitan who left him alone.
For two days, he received the two dinars and utilized them, but on the third day, there was no sign of the money. Upset and distressed, he picked up his axe and set out to cut down the tree.
On the way, he encountered Shaitan, who asked him: "Where are you headed for?"
"I am going to cut that tree."
"There is no way you are going to do it," said Shaitan.
Once again they began to contest, but this time Iblees overcame him and hurling him to the ground, ordered, "Turn back or I shall sever your head from your body."
The pious man said, "Leave me alone and I shall return, but tell me, how was it that I hadmanaged to overcome you on the previous occasion?"
Iblees answered, "On that occasion, you had set out only for Allah and you were sincere in your intention as a result of which, Allah subjugated me for you, but this time you were angry for your own self and for your dinars and so I could overpower you." 
4 - The Secret of a Sincere Slave
Sa’eed Ibn Musayyab narrates:
"One year, there was a severe famine and so the people gathered together to pray for the rains. I looked around and my eyes fell upon a black slave, who had separated himself from the crowd and emerged on top of a small hill. I advanced in his direction and when I came near him, I noticed that his lips were moving in prayer. Hardly had he completed his prayers, when a cloud appeared in the sky.
Seeing the cloud, the black slave praised Allah and moved away. Very soon, rains lashed us so heavily that we thought we might perish.
I ran after the slave and observed that he entered the house of Imam Sajjad (a.s.). I arrived before the Imam (a.s.) and said:
"O’ My Master! In your house there is a black slave; oblige me by selling him to me."
He (a.s.) replied, "O’ Sa’eed! Why should I not gift him to you, instead,"and ordered the head of his slaves to bring all the servants before me. When they had assembled, I noticed that the black slave was not amongst them.
I said, "The one whom I desire, is not amongst them."
The Imam (a.s.) said, "There remains no other slave except one."
He then ordered him to be brought forth. When the slave was brought before me, I saw that he was the very person whom I had sought and so I said, "He is the one I need."
"O’ Slave! Henceforth, Sa’eed is your master so go with him," instructed the Imam (a.s.).
The slave turned to me and asked, "What prompted you to separate me from my master?" 
I replied, "When I witnessed your prayers for the rains being accepted, I wished that I could ownyou."
When he heard this, the slave stretched out his hands in supplication and, turning his face towards the skies, beseeched:
"O’ My Lord! This was a secret between You and me. Now that You have divulged it, grant me death and take me towards Yourself."
The Imam (a.s.) and all those present, wept over the position of the slave, while I, shedding tears, came out of the house. No sooner had I reached my own house, than the Imam’s (a.s.) messenger arrived and said, "Come along if you wish to takepart in the funeral procession of your companion."
I returned to the Imam’s (a.s.) house along with the messenger only to find that the slave had passed away.
5 - Prophet Musa’s (a.s.) Request
Prophet Musa (a.s.) once requested to God:
"O’ Lord! It is my wish to see that creature of Yours who has purified himself for your worship and who is unpolluted in his obedience towards You."
He was addressed, "O’ Musa! Go near the shores of such-and-such sea in order that I may showyou what you desire to see."
Prophet Musa proceeded till he reached near the sea. Looking around, he observed that on a branch of a tree that drooped over the water, sat a bird, engrossed in the dhikr of Allah. When Musa questioned the bird about itself, the bird said:
"From the time Allah has created me, I have been on this branch, engaged in His worship and dhikr. From every dhikr of mine, there branch out a thousand other dhikr, and the pleasure which I derive from the dhikr of Allah, provides me with nourishment."
"Do you crave anything from this world?" asked Musa (a.s.).
"Yes. I yearn to taste one drop of water from this sea," replied the bird.
Musa (a.s.) exclaimed, "But there does not exist a great distance between your beak and the water! Why don’t you dip your beak into it and drink it?"
The bird answered, "Out of fear lest the enjoyment derived from the water should make me heedless of the pleasure of the dhikr of my Lord."
Hearing this, Prophet Musa (a.s.) clasped his head in intense astonishment.
 Holy Qur’an, ch. Al-Zumar (39), vs. 2.
 Jaame’ al-Sa’adaat, vol. 2, pg. 404.
 Tadhkirah al-Haqaaiq, pg. 73.
 In the book Mahaasin, it has been mentioned that his wage was half a dirham but when he returned to collect it, he was given eighteen thousand times over!
 Namunah-e-Ma’arif, vol. 1, pg. 53; Farajun Ba’d al-Shiddah, pg. 23; Mahaasin-e-Barqi, vol. 2, pg. 253.
 Mowlana, in his Mathnawi, has composed a poem over the incident, which is as follows:
از علي آموز اخلاص عمل شير حق را دان منزه از دغل
در غزا بر پهلواني دست يافت زود شمشير برآورد و شتافت
او خدو انداخت بر روي علي افتخار هر نبي و هر ولي
ضَرْبَةُ عَلِيٍّ يَوْمَ خَنْدَق أَفْضَلُ مِنْ عِباَدَةِ الثَّقَلَيْن
 Pand-e-Taareekh, vol. 5, pg. 199; Anwaar al-Nu’maaniyyah; A’in al-Hayaah.
 Iblees means the 'carnal soul’, while the pious worshipper alludes to the 'pure innate and soul’.
 Namunah-e-Ma’arif, vol. 1, pg. 54; Ihyaa al-U’loom, vol. 4, pg. 380; Riyadh al-Hikaayaat, pg. 140.
ماَ حَمَلَكَ عَلَي أَنْ فَرَّقْتَ بَيْنِي وَ بَيْنَ مَوْلاَيَ
 Muntahal Aa’maal, vol. 2, pg. 38; Ithbaat al-Wasiyyah (author: Masu’di).
 Khazinah al-Jawaahir, pg. 318