Sayyiduna 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 necessary for every Muslim. Whoever prefers to pray five rak'ahs of witr, let him do so; whoever prefers to pray three rak'ahs of witr, let him do so; and whoever prefers to pray one rak'ah of witr, let him do so."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4776