Musnad Ahmad 27:1108

Yazid ibn Harun narrated to us, Sufyan ibn Sa'id informed us, 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 on a donkey, while it was facing the east, performing a voluntary prayer.