Musnad Ahmad 5:364

'Abd al-Razzaq narrated to us, Ma'mar and al-Thawri narrated to us, from Abu Ishaq, from 'Asim ibn Damrah, from 'Ali, who said: "The Witr is not an obligation like the prescribed prayer, but it is a Sunnah that the Messenger of Allah, may Allah bless him and grant him peace, established."