Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 536
Narrated Ibn Abbas:
I do not know whether the Prophet forbade the eating of donkey-meat (temporarily) because they were the beasts of burden for the people, and he disliked that their means of transportation should be lost, or he forbade it on the day of Khaibar permanently.