Sahih Muslim Book 21, Hadith Number 4774.

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

Ibn ‘Abbas reported: I do not know whether Allah’s Messenger (may peace be upon him) prohibited (the eating of the domestic ass) due to the fact that they were the beasts of burden for the people, so he (the Holy Prophet) did not like their beasts of burden to be destroyed (as a matter of expediency), or he prohibited the use of the flesh of domestic asses (not as an expediency but as a law of the Shari’ah) on the Day of Khaibar.