Abu Dawud Book 2, Hadith Number 1417.
Chapter : How many rakats are there in the witr.
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.