Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) 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 (saw) 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).'" Daif