Sahih Bukhari Volume 004, Book 053, Hadith Number 383.

Narated By Ibn Abi Aufa : We were afflicted with hunger during the besiege of Khaibar, and when it was the day of (the battle of) Khaibar, we slaughtered the donkeys and when the pots got boiling (with their meat). Allah's Apostle made an announcement that all the pots should be upset and that nobody should eat anything of the meat of the donkeys. We thought that the Prophet prohibited that because the Khumus had not been taken out of the booty (i.e. donkeys); other people said, "He prohibited eating them for ever." The sub-narrator added, "I asked Said bin Jubair who said, 'He has made the eating of donkeys' meat illegal for ever.")