(a) It is narrated from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) that he said: "Witr is not obligatory; it is a Sunnah. the Messenger of Allah (peace and blessings be upon him) established it as a Sunnah."
(b) This is the view of Sayyiduna Ubadah bin al-Samit and Sayyiduna Ibn Abbas (may Allah be pleased with them both). The other narrations that have come on this subject will be mentioned in the chapter on voluntary prayer, because that is their proper place.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2227