Muwatta Malik Book 7, Hadith 16
Yahya related to me from Malik from Yahya ibn Said that Said ibn
al-Musayyab said, "Abu Bakr as-Siddiq used to pray witr when he wished
to go to bed, and Umar ibn al-Khattab used to pray witr at the end of
the night. As for me, I pray witr when I go to bed."
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.' "
Yahya related to me from Malik that he had heard that a man asked Abdullah ibn Umar whether the witr was obligatory and Abdullah ibn Umar said, "The Messenger of Allah, may Allah bless him and grant him peace, prayed witr, and the muslims prayed witr." The man began repeating his question, and Abdullah ibn Umar kept saying, "The Messenger of Allah, may Allah bless him and grant him peace, prayed witr, and the muslims prayed witr."