Musannaf Ibn Abi Shayba 3:4389

Yazid ibn Harun narrated to us, from Hisham al-Dastawa'i, from Hammad, from Ibrahim, from Abu Abdullah al-Jadali, from Abu Mas'ud, who said: "The Messenger of Allah (may Allah bless him and grant him peace) performed the Witr prayer at every part of the night: at its beginning, its middle, and its end. His Witr prayer concluded at the time of the dawn (al-sahar)."