Musnad Abu Ya'la al-Mawsili 154:34

'Abd al-A'la ibn Hammad narrated to us, Wahib narrated to us, 'Amr ibn Yahya narrated to us, from Sa'id ibn Yasar, from 'Abdullah ibn 'Umar, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) praying upon a donkey while he was heading toward Khaybar.