Narrated AbuQatadah:

The Prophet asked AbuBakr: When do you observe the witr?

He replied: I observe the witr prayer in the early hours of the night.

The Prophet 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.