Musnad Ahmad 5:653

'Abdullah narrated to us, 'Ali bin Hakim al-Awdi narrated to us, Sharik informed us, from Abu Ishaq, from 'Asim bin Damrah, from 'Ali, who said: "The Witr is not an obligation, but it is a Sunnah that the Messenger of Allah, peace and blessings of Allah be upon him, established."