Maliks Muwatta Book 7, Hadith Number 15.

Section : The Command to Pray the Witr.

Yahya related to me from Malik from Abu Bakr ibn Umar that Said ibn Yasar said, ”I was travelling with Abdullah ibn Umar on the road to Makka, and fearing that it was nearly dawn. I dismounted and prayed witr. Abdullah said, ‘Is there not a model for you in the Messenger of Allah, may Allah bless him and grant him peace?’ I said, ‘Of course, by Allah!’ He said, ‘The Messenger of Allah, may Allah bless him and grant him peace, used to pray witr on his camel.'”