Musnad Abu Ya'la al-Mawsili 153:174

Ubayd Allah ibn Mu'adh ibn Mu'adh al-'Anbari narrated to us, my father narrated to us, Shu'bah narrated to us, from Khubayb ibn 'Abd al-Rahman, from Hafs ibn 'Asim, from Ibn 'Umar, from the Prophet, may Allah bless him and grant him peace: That he used to pray upon his riding beast wherever it faced him.