Sahih Muslim Book 21, Hadith Number 4777.

Chapter : The eating of the flesh of domestic asses is unlawful.

Anas reported: When Allah’s Messenger (may peace be upon him) conquered Khaibar, we caught hold of the asses outside the village. We cooked them (their flesh). Then the announcer of Allah’s Messenger (may peace be upon him) made the announcement: Listen, verily Allah and His Messenger have prohibited you (the eating of) their (flesh), for it is a loathsome evil of Satan’s doing. Then the earthen pots were turned over along with what was in them, and these were brimming (with flesh) at that time.