Sahih Bukhari: Hadith 4055

We where afflicted with severe hunger on the day of Khaibar. While the cooking pots were boiling and some of the food was well-cooked, the announcer of the Prophet (ﷺ) came to say, "Do not eat anything the donkey-meat and upset the cooking pots." We then thought that the Prophet (ﷺ) had prohibited such food because the Khumus had not been taken out of it. Some others said, "He prohibited the meat of donkeys from the point of view of principle, because donkeys used to eat dirty things."