It is narrated from Sayyiduna Abu Tha'labah (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) was asked: "Is there zakah on donkeys?" He (peace and blessings be upon him) said: "No, except for this unique and singular verse: 'Whoever does an atom's weight of good will see it.'"
Hadith Referenceالمعجم الكبير للطبراني / باب اللام ألف / 18822