Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: I do not know whether the Messenger of Allah (peace and blessings be upon him) forbade it because the people used it for riding, lest their mounts be used up, or whether on the day of Khaybar he declared the meat of the domestic donkey unlawful.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19460