Musnad Abu Ya'la al-Mawsili 154:17

Ishaq ibn Abi Isra'il narrated to us, Abdullah ibn al-Mubarak narrated to us, from Abd al-Malik ibn Abi Sulayman, from Sa'id ibn Jubayr, from Ibn Umar, that he used to pray upon his riding beast wherever it faced him. And he mentioned that the Messenger of Allah (peace be upon him) used to pray upon his riding beast wherever it faced him. And Ibn Umar recited: "So wherever you [might] turn, there is the Face of Allah" (Al-Baqarah).