Musnad Ahmad 5:90

Aswad ibn Amir and Husayn narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from al-Harith, 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: at its beginning, its middle, and its end. Then, the Witr became established at the end of the night."