Musnad Ahmad 32:1166

Yazid ibn Harun narrated to us, he said: Rabi' ibn al-Jarud ibn Abi Sabrah al-Tamimi informed us, he said: 'Amr ibn Abi al-Hajjaj narrated to me, from al-Jarud ibn Abi Sabrah, from Anas ibn Malik, who said: "When the Messenger of Allah, peace and blessings of Allah be upon him, wanted to pray a voluntary prayer upon his riding beast, he would face the Qibla, then say the takbir for the prayer, then let his riding beast go, and pray wherever it turned with him."