Musnad Ahmad 5:199

'Abd al-Rahman ibn Mahdi narrated to us, Sufyan narrated to us, from Abu Ishaq, from 'Asim ibn Damrah, from 'Ali, who said: "The Witr is not an obligation like the form of the [prescribed] prayer, but it is a Sunnah that the Messenger of Allah, peace and blessings of Allah be upon him, established."