It is narrated from Sayyiduna Abu Ayyub (may Allah be pleased with him) that the Messenger of Allah (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 / 3951