Musnad Ahmad 33:921

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Yahya ibn Abi Kathir, from Muhammad ibn 'Abd al-Rahman ibn Thawban, from Jabir ibn 'Abd Allah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray the voluntary prayer upon his riding beast wherever it faced him during travel. But when he wanted to pray the obligatory prayer, he would descend from his riding beast and face the Qibla."