Musnad Ahmad 842:1

Yazid ibn Harun narrated to us, Jarir ibn Hazim informed us, al-Hasan narrated to us, from Sa'sa'ah ibn Mu'awiyah, the paternal uncle of al-Farazdaq, that he came to the Prophet, peace and blessings of Allah be upon him, and he recited to him: "So whoever does an atom's weight of good will see it, and whoever does an atom's weight of evil will see it." He said: "That is sufficient for me; I do not care if I do not hear anything else."