Musnad Ahmad 27:759

Waki’ narrated to us, Malik ibn Anas narrated to us, from Abu Bakr ibn ‘Umar, from Sa’id ibn Yasar, who said: Ibn ‘Umar said to me: "Do you not have an example in the Messenger of Allah?" "The Messenger of Allah (peace and blessings of Allah be upon him) used to perform the Witr prayer while on his camel."