Musnad Ahmad 27:533

Abu Ahmad al-Zubayri narrated to us, Sufyan narrated to us, from Mansur, from Abd al-Rahman ibn Sa’d, who said: I was with Ibn Umar, and he was praying upon his riding beast [facing] this way and that way. I said to him [about that], and he said: "Thus I saw the Messenger of Allah, may Allah bless him and grant him peace, doing."