Prayer (Kitab Al-Salat): Detailed Injunctions about Witr
Dawud :: Book 8 : Hadith 1417
Narrated AbuAyyub al-Ansari:
The Prophet (peace_be_upon_him) 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.