Musnad Ahmad 33:418

Isma'il narrated to us, Hisham al-Dastawa'i informed us, from Yahya ibn Abi Kathir, from Muhammad ibn 'Abd al-Rahman, from Jabir ibn 'Abd Allah, who said: "The Messenger of Allah (may Allah bless him and grant him peace) used to pray upon his riding beast facing the east. But when he wanted to pray the obligatory prayer, he would descend and face the Qibla."