Sahih Muslim Book 21, Hadith Number 4769.

Chapter : The eating of the flesh of domestic asses is unlawful.

Sulaiman Shaibini reported: I heard Abdullah b. Abu Aufa say: We were smitten with hunger during the nights of Khaibar. On the Day of Khaibar, we fell upon domestic asses and we slaughtered them, and when our earthen pots boiled with them, the announcer of Allah’s Messenger (may peace be upon him) made an announcement that the earthen pots should be turned over, and nothing should be eaten of the flesh of the domestic asses. Some of the people said that Allah’s Messenger (may peace be upon him) had forbidden (the use of this flesh) for one-fifth (due to the State) has not been paid, while others said: He prohibited it for ever.