Asim bin Damrah narrates from Sayyiduna Ali (may Allah be pleased with him) that witr is not obligatory, but it is a good Sunnah of the Prophet (peace and blessings be upon him), because "Allah, the Lord of Might, is Witr (One) and He loves witr."
(b) These are the words of Zuhayr's narration; in al-Thawri's narration the words are: Witr is not obligatory, rather it is a Sunnah; the Messenger of Allah (peace and blessings be upon him) established it as a Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4453