Musannaf Ibn Abi Shayba 3:4378

Abu Bakr ibn Ayyash narrated to us, from Abu Husayn, from Yahya ibn Waththab, from Masruq, who said: I asked Aisha about the Witr of the Messenger of Allah, may Allah bless him and grant him peace. She said: "He performed Witr during every part of the night. The Messenger of Allah, may Allah bless him and grant him peace, performed it in the beginning of it, the middle of it, and his Witr ended at the time of his death in the last part of the night (al-sahar)."