It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) (the narrator attributed it to the Prophet (peace and blessings be upon him) as a marfu' report) that he (peace and blessings be upon him) said: "Witr is obligatory upon every Muslim. Whoever is able to pray five rak'ahs of witr, let him do so; whoever cannot pray five, let him pray three; whoever cannot pray three, let him pray one; and whoever cannot pray even one, let him pray it by gestures."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3949