Musnad Ahmad 5:92

Waki’ narrated to us, Shu’bah narrated to us, from Abu Ishaq, from ‘Asim ibn Damrah, from ‘Ali, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, performed Witr at the beginning of the night, at its end, and in its middle. Then, his Witr concluded at the time of the dawn (Sahar)."