Musnad Ahmad 5:693

Abdullah narrated to us, Yahya ibn Adawayh Abu Muhammad, the freed slave of Banu Hashim, 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 the end of the night."