Musnad Abu Ya'la al-Mawsili 3:337

Abu Khaythama narrated to us, Muhammad ibn Fudayl narrated to us, from Mutarrif, from Abu Ishaq, from Asim ibn Damra, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to perform the Witr prayer at the beginning of the night, in the middle of it, and at the end of it. Then, his Witr prayer became established at the end of it.