Musnad Ahmad 5:279

Muhammad ibn Ja'far narrated to us, from Shu'bah, from Abu Ishaq, who said: I heard 'Asim ibn Damrah narrating from 'Ali, who said: "The Witr is not an obligation like the prayer, but it is a Sunnah, so do not abandon it." Shu'bah said: "And I found it written with me: 'And the Messenger of Allah, peace and blessings of Allah be upon him, performed Witr.'"