Abu Dawud Book 2, Hadith Number 1414.
Chapter : On a person who does not observe the witr prayer.
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.