Musnad Abu Ya'la al-Mawsili 154:37

Zuhayr narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Malik ibn Anas, from Abu Bakr, from Sa'id ibn Yasar, from Ibn Umar: That the Messenger of Allah (peace and blessings of Allah be upon him) performed the Witr prayer while on a camel.