Abu Ayyub al-Ansari (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Witr is established (it is a duty). Whoever wishes may pray seven rak'ahs of witr, whoever wishes may pray three rak'ahs of witr, and whoever wishes may pray one rak'ah of witr; and whoever is unable even to do that should perform it by gestures."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4779