It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Witr is a truth; whoever wishes may pray seven rak'ahs of witr, whoever wishes may pray five rak'ahs, whoever wishes may pray three rak'ahs, and whoever wishes may pray one rak'ah."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3950