Musnad Ahmad 5:586

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Ishaq, from 'Asim ibn Damrah, from 'Ali, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, performed the Witr prayer during every part of the night: in its beginning, its middle, and its end. And his Witr concluded at its end."