Musnad Ahmad 1153:1675

Waki’ narrated to us, from Sufyan, from ‘Asim ibn Abi al-Najud, from Abu al-Duha, from Masruq, from ‘Aisha, who said: "The Messenger of Allah (may Allah bless him and grant him peace) performed Witr during every part of the night: in its beginning, its middle, and its end. His Witr eventually concluded at the time of the predawn (al-sahar), and he passed away while performing Witr at the time of the predawn."