Musnad Abu Ya'la al-Mawsili 3:57

Ubayd Allah narrated to us, Ghundar narrated to us, from Shu'bah, from Abu Ishaq, who said: I heard 'Asim ibn Damrah narrating from 'Ali, that he said: "The Witr is not an obligation like the (obligatory) prayer, but it is a Sunnah, so do not abandon it." Shu'bah said: And I found it written with me: "For the Messenger of Allah (ﷺ) used to perform the Witr."