Musnad Ahmad 27:598

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, from Abd al-Rahman ibn Sa'd, who said: I accompanied Ibn Umar from Medina to Mecca. He began to pray upon his riding beast facing the direction of Mecca. I said to Salim: "If his face were toward Medina, how would he pray?" He said: "Ask him." So I asked him, and he said: "Yes, and here, and here," and he said: "Because the Messenger of Allah, may Allah bless him and grant him peace, did it."