Musnad Abu Ya'la al-Mawsili 154:156

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