Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "Witr is a duty (haqq). Whoever wishes may pray five or three rak'ahs of witr, and whoever wishes to pray one may pray one."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4777