Abu Dawud Book 2, Hadith Number 1429.
Chapter : On observing the witr before sleeping.
Narated By Abu Qatadah : The Prophet (PBUH) asked Abu Bakr: When do you observe the witr?
He replied: I observe the witr prayer in the early hours of the night.
The Prophet (PBUH) asked Umar: When do you observe the witr?
He replied: At the end of the night.
He then said to AbuBakr: This has followed it with care; and he said to Umar: He has followed it with strength.