Musnad Ahmad 5:263

'Affan narrated to us, Shu'bah narrated to us, he said: Abu Ishaq informed me more than once, he said: I heard 'Asim ibn Damrah, from 'Ali, that he said: "The Messenger of Allah (ﷺ) performed the Witr prayer during every part of the night: in its beginning, its middle, and its end; and his Witr concluded at the end of the night."