Tafsir Al-Jalalayn: Az-Zalzala, Ayah 7

Allah then revealed the following verse about a group of people who thought that they are not rewarded for any small goodness they do nor punished for any small evil they commit, and so Allah urged them to do acts of goodness even if they are small and warned them against acts of evil even if they are small acts: (And whoso doeth good an atom's weight) the weight of the smallest ant (will see it then) in his register and then will be happy with it; it is also said: the believer sees his works in the Hereafter while the disbeliever sees his works in the life of the world,