Musnad Ahmad 5:405

Abu Mu'awiyah narrated to us, Hajjaj narrated to us, from Abu Ishaq, from 'Asim ibn Damrah, from 'Ali, who said: He was asked about the Witr: "Is it obligatory?" He said: "As for like the obligatory prayer, then no. But it is a Sunnah that the Messenger of Allah, peace and blessings of Allah be upon him, and his companions practiced, until they passed on while upon that."