It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that a man came to the Messenger of Allah (peace and blessings be upon him) and said, "Donkeys are being eaten." Then another came and said, "The donkeys are about to be finished." So the Noble Prophet (peace and blessings be upon him) ordered a caller to announce: "Indeed, Allah Almighty and His Messenger (peace and blessings be upon him) forbid you from the meat of domestic donkeys; they are impure." The narrator says the boiling pots were overturned.
Hadith Referenceالسنن المأثورة / باب في أكل لحوم الخيل والبغال والحمير / 572