Maliks Muwatta Book 7, Hadith Number 17.
Section : The Command to Pray the Witr.
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.”