Musnad Ahmad 27:1611

Sulayman ibn Hayyan, Abu Khalid al-Ahmar, narrated to us, from Ubayd Allah—meaning Ibn Umar—from Nafi, from Ibn Umar, who said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, praying upon his riding beast." And Nafi [narrated] that Ibn Umar used to pray upon his riding beast.