Sunan Ibn Majah, Chapter: The Chapters on Slaughtering

"I asked Abdullah bin Abu Awfa about the flesh of domesticated donkeys and he said: 'We were starving on the Day of Khaibar, when we were with the Prophet (saw). The people had gotten some donkeys as spoils of war on the way out from Al-Madinah, so we slaughtered them and our cooking pots were boiling when the caller of the Messenger of Allah (saw) cried out, telling us to overturn out pots and not to eat anything of the flesh of donkeys. So we overturned them.' I said to Abdullah bin Abu Awfa: 'Was it made unlawful?' He said: 'We think that the Messenger of Allah (saw) forbade it altogether because it eats excrement.'"