Musnad Abu Ya'la al-Mawsili 153:45

Abu Khaythama narrated to us, Yunus ibn Muhammad narrated to us, Abdullah ibn Umar narrated to us, from Musa ibn Uqbah, from Salim, from his father: That the Messenger of Allah (peace and blessings of Allah be upon him) used to perform the Witr prayer while on his riding beast.