Musnad Ahmad 27:650

'Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, 'Amr ibn Yahya al-Mazini al-Ansari narrated to me, Sa'id ibn Yasar narrated to me, from Ibn 'Umar, who said: "I saw the Prophet, peace and blessings of Allah be upon him, praying upon a donkey while he was heading toward Khaybar."