It is narrated from Sayyiduna Abu Ayyub (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Witr is a truth; whoever wishes to pray five rak'ahs of witr, let him do so; whoever wishes to pray three rak'ahs, let him do so; and whoever wishes to pray one rak'ah, let him do so."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3947