Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1414
Narated By Buraydah ibn al-Hasib : I heard the Apostle of Allah (pbuh) 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.