Sahih Muslim
The Book of Games and the Animals which May be Slaughtered and the Aninals that Are to be Eaten (Kitab-us-Said wal-Dhabaih wa ma Yukalu min Al-Hayawan)
Book Number: 021 / Hadith Number: 4767
Ibn 'Umar reported that Allah's Messenger (may peace be upon him) forbade the eating of the (flesh) of domestic asses on the Day of Khaibar in spite of the fact that people needed that.