Musannaf Ibn Abi Shayba 16:103

Abu Bakr narrated to us, he said: Ali bin Mushir narrated to us, from al-Shaybani, he said: I asked Abdullah bin Abi Awfa about the meat of domestic donkeys. He said: A famine struck us on the day of Khaybar while we were with the Messenger of Allah, may Allah bless him and grant him peace. We had obtained donkeys for the people outside of the city, so we slaughtered them. Our pots were boiling when the announcer of the Messenger of Allah, may Allah bless him and grant him peace, called out: "Overturn the pots and do not eat anything of the meat of the donkeys." I said: "Did he prohibit them with the prohibition of what?" He said: We talked among ourselves and said: "He prohibited them absolutely," or "He prohibited them because they were not included in the one-fifth (khums)."