Musnad Ahmad 33:160

Isma'il—meaning Ibn 'Ulayyah—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 (peace and blessings of Allah be upon him) 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 Qiblah."