Muwatta Malik Book 7, Hadith 17
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."
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 that he had heard that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, used to say, "If someone fears that he will sleep through till the morning, let him pray the witr before he sleeps, and if some one hopes to wake for the last part of the night, let him delay his witr."