Musnad Ahmad 30:2349

Abu Mu'awiyah narrated to us, he said: Suhayl narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, was asked about donkeys: Is there Zakat in them? He said: "Nothing has come to me regarding them except this singular verse: {So whoever does an atom's weight of good will see it, and whoever does an atom's weight of evil will see it}."