Musnad Ahmad 215:7

Abu Dawud al-Tayalisi narrated to us, he said: Harb ibn Shaddad narrated to us, from Yahya ibn Abi Kathir, from al-Nahhaz al-Hanafi, that Sinan ibn Salamah informed him, from his father: "The Messenger of Allah (peace and blessings of Allah be upon him) ordered that the meat of domestic donkeys be [discarded] on the day of Khaybar, while it was in the cooking pots, so it was overturned."