Musnad Ahmad 27:1376

'Abd al-Wahhab narrated to us, Hisham informed us, from Hammad, from 'Abd al-Rahman ibn Sa'd, the freed slave of 'Umar ibn al-Khattab, that he saw 'Abd Allah ibn 'Umar praying upon his riding beast facing other than the Qibla for a voluntary prayer. He said: "What is this, O Abu 'Abd al-Rahman?" He said: "The Prophet of Allah, may Allah bless him and grant him peace, used to do it."