In the Holy Qur'an

So, he has done an atom's weight of good shall see it. And he who has done an atom's weight of evil should see it. (99:7-8)

In the Traditions

Imam Jafar was asked the meaning of Allah's words, '(In order) that He may try you - which of you is best in deed' (67:2) and he answered, "this does not mean the greatest number of deeds, but rather the most correct deeds: truly, correctness comes from fear of Allah and a good and sincere intention. Perseverance in a deed until one finishes it is the hardest of deeds. A sincere deed is one for which you want no one's praise but Allah's; indeed the intention is better than the deed itself, rather, the intention is a deed."

Then he read Allah's word, "Say: Everyone acts according to his manner" (17:84) and explained it as meaning, "according to his intention."