Musnad Ahmad 27:552

Abdullah ibn Idris narrated to us, from Abd al-Malik—meaning Ibn Abi Sulayman—from Sa'id ibn Jubayr, from Ibn Umar: "He would pray wherever his riding beast turned him, and I have seen the Messenger of Allah (peace and blessings of Allah be upon him) do that." And he would interpret regarding it: "And wheresoever you are, turn your faces (in prayer) towards it" [Al-Baqarah: 144].