Musannaf Ibn Abi Shayba 3:4385

Hushaym narrated to us, he said: Mutarrif informed us, from Abu Ishaq, from some of the companions of Ali, who said: Ali said: "From every part of the night, the Messenger of Allah (may Allah bless him and grant him peace) performed the Witr prayer: from its beginning, its middle, and its end. However, the Witr prayer was established for the Messenger of Allah (may Allah bless him and grant him peace) at the end of the night."