Musannaf Ibn Abi Shayba 3:6135

Yazid ibn Harun narrated to us, from Rib'i ibn Abdullah ibn al-Jarud ibn Abi Sabra al-Tamimi, who said: Amr ibn Abi al-Hajjaj narrated to me, from al-Jarud ibn Abi Sabra, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace), when he wanted to pray a voluntary prayer upon his riding beast, would face the Qibla and say the Takbir for the prayer. Then, he would let his riding beast go, and he would pray wherever it turned with him.