Musnad Ahmad 32:736

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ayyub, from Ibn Sirin, from Anas, that the herald of the Messenger of Allah (peace and blessings of Allah be upon him) proclaimed: "Indeed, Allah and His Messenger forbid you from eating the meat of domesticated donkeys, for it is an abomination."