Ata' bin Yazid al-Laythi (may Allah have mercy on him) narrated from Abu Ayyub al-Ansari (may Allah be pleased with him): "Witr is a duty. Whoever wishes to pray five rak'ahs of witr may pray them; whoever wishes to pray three rak'ahs of witr may pray three; whoever wishes to pray one rak'ah of witr may pray one; and whoever is not able should perform it by gestures of his head; he may do so."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4795