Musnad Ahmad 1153:1676

Waki' and Abd al-Rahman narrated to us, they said: Sufyan narrated to us, from Abu Hasin, from Yahya ibn Wathab, from Masruq, from Aisha, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, performed Witr during every part of the night: in its beginning, its middle, and its end. His Witr concluded at the time of the dawn (al-sahar)."