Asim bin Damrah narrates from Sayyiduna Ali (may Allah be pleased with him) that witr is not obligatory like the obligatory prayer; rather, it is a Sunnah. The Prophet (peace and blessings be upon him) established it as a Sunnah. He said: "O people of the Qur'an, pray witr, for indeed Allah Almighty is Witr (One) and He loves witr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4454