Sahih Muslim
The Book of Games and the Animals which May be Slaughtered and the Aninals that Are to be Eaten (Kitab-us-Said wal-Dhabaih wa ma Yukalu min Al-Hayawan)
Book Number: 021 / Hadith Number: 4775
Salama b. Akwa' reported: We went to Khaibar with Allah's Messenger (may peace be upon him). Then Allah granted (us) victory over them. On that very evening of the day when they had been granted victory, they lit many fires. Thereupon Allah's Messenger (may peace be upon him) said: What are those fires and what for those have been lit? They said: (These have been lit) for (cooking) the flesh. Thereupon he said: Of what flesh? They said: For the flesh of the domestic asses. Thereupon Allah's Messenger (may peace bo upon him) said: Throw that away and break them (the earthen pots in which the fiesa was being cooked). A person said: Messenger of Allah, should we throw it away and wash them (the cooking pots)? He said: You may do so.