Musnad Abu Ya'la al-Mawsili 154:36

Abu Khaythama narrated to us, Abd al-Rahman narrated to us, from Malik, from Amr ibn Yahya, from Sa'id ibn Yasar, from 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.