Musnad Ahmad 27:1660

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, from 'Amr—meaning ibn Yahya—from Sa'id ibn Yasar, from 'Abdullah ibn 'Umar, who said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, praying upon a donkey, while he was heading toward Khaybar."