Musnad Ahmad 5:19

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