سنن ابن ماجه

It was narrated that Jabir bin ‘Abdullah said: “The Messenger of Allah (ﷺ) said to Abu Bakr: ‘When do you pray Witr?’ He said: ‘At the beginning of the night, after ‘Isha’.’ He said: ‘And you, O ‘Umar?’ He said: ‘At the end of the night.’ The Prophet (ﷺ) said: ‘As for you, O Abu Bakr, you have seized the trustworthy handhold (i.e., you want to be on the safe side), and as for you, O ‘Umar, you have seized strength (i.e., you are confident that you have the resolve to get up and pray Witr).’” Another chain with similar meaning.