Musnad Ahmad 1153:957

Aswad ibn Amir narrated to us, he said: Abu Bakr ibn Ayyash informed us, from Asim, from Muslim, from Masruq, and Abu Husayn, from Yahya ibn Waththab, from Masruq, who said: I asked Aisha about the Witr of the Prophet, may Allah bless him and grant him peace. She said: "He performed Witr during every part of the night; its middle, its end, and its beginning. His Witr eventually concluded at the time of the predawn (Sahar) until he died."