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: 4779
Jabir b. 'Abdullah reported that Allah's Messenger (may peace be upon him) prohibited eating of the flesh of domestic asses on the Day of Khaibar, and permitted the cooking of the flesh of horses.