Musnad Ahmad 27:591

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Khubayb ibn 'Abd al-Rahman, from Hafs ibn 'Asim, from Ibn 'Umar, that he used to pray wherever his riding beast turned him. He said: "And the Messenger of Allah, may Allah bless him and grant him peace, used to do that."