Sunan Abi Dawud

Narrated Buraydah ibn al-Hasib: I heard the Messenger of Allah (ﷺ) say: The witr is a duty, so he who does not observe it does not belong to us; the witr is a duty, so he who does not observe it does not belong to us; the witr is a duty, so he who does not observe it does not belong to us