Musnad Abu Ya'la al-Mawsili 123:5

Muhammad ibn Ishaq al-Musayyibi narrated to us, Sulayman ibn Dawud ibn Qays narrated to me, from Dawud ibn Qays, from Yahya ibn Sa'id, from Anas ibn Malik, that he saw the Messenger of Allah (may Allah bless him and grant him peace) while he was praying upon a donkey, and he was going to Khaybar, and the qiblah was behind him.