Musnad Ahmad 32:337

'Abd al-Samad ibn 'Abd al-Warith narrated to us, Bakkar ibn Mahan narrated to us, Anas ibn Sirin narrated to us, from Anas ibn Malik: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray voluntary prayers on his she-camel while traveling, facing other than the Qibla."