Musnad Ahmad 5:651

'Abdullah narrated to us, Abu Khaythama narrated to me, Muhammad bin Fudayl narrated to us, from Mutarrif, from Abu Ishaq, from 'Asim bin Damra, 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, and in its middle, and in its end. Then, his Witr became established at its end."