Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1417
Narated By Abu Ayyub al-Ansari : The Prophet (pbuh) said: The witr is a duty for every Muslim so if anyone wishes to observe it with five rak'ahs, he may do so; if anyone wishes to observe it with three, he may do so, and if anyone wishes to observe it with one, he may do so.