Sayyiduna Abdullah bin Abi Awfa (may Allah be pleased with him) says: During the days of Khaybar we were struck by severe hunger, and we came upon domestic donkeys, which we slaughtered. When the pots were boiling, the caller of the Messenger of Allah (peace and blessings be upon him) announced: "Overturn the pots, and do not eat the meat of the donkeys." The narrator says: Some people said: The Messenger of Allah (peace and blessings be upon him) forbade it because the one-fifth (khumus) had not been taken from it; and others said: He (peace and blessings be upon him) forbade it for good.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19461