So whosoever does good equal to the weight of an atom shall see it.
And whosoever does evil equal to the weight of an atom shall see it.
[99:7-8]

"It is reported on the authority of Sa`id ibn Jubayr: When the verse: "And they give food in spite of their love for it, to the poor, the orphan, and the captive," (76:8) was revealed, the people thought that it would carry no weight if they give some paltry thing in alms. Consequently, they started turning back beggars thinking the alms of a date or a piece of bread to be of no value. They thought that they should give something valuable in alms if they could afford it. On the other hand there was a group who thought that they would not be called to account for small sins, like telling a lie. They thought that the warning of the Hell is for major sins. To correct false impressions these verses were revealed.

The Prophet sallallahu `alaihi wa sallam said, "Do not understate sin. They gather together to destroy a person. (He set forth a parable of small sins). They are as some people pick up one or two small pieces of wood each. These pieces, put together, will make a heap which if lit would produce enough fire to cook anything."