Abu Dawud Book 2, Hadith Number 1430.

Chapter : On the time of witr prayer.

Narated By Masruq : I asked ‘Aisha : When would the Apostle of Allah(PBUH) observe the witr prayer? She replied: Any time he observed the witr, sometimes in the early hours of the night, sometimes at midnight and sometimes towards the end of it. But he used to observe the witr just before the dawn when he died.