Musannaf Ibn Abi Shayba 3:4331

Husayn ibn Ali narrated to us, from Za'idah, from Abdullah ibn Muhammad ibn Aqil, from Jabir ibn Abdullah, who said: The Messenger of Allah (peace be upon him) said to Abu Bakr: "When do you perform the Witr prayer?" He said: "At the beginning of the night, after the night prayer (al-'atamah), before I sleep." And he said to Umar: "When do you perform the Witr prayer?" He said: "At the end of the night." He said to Abu Bakr: "You have taken the cautious approach." And he said to Umar: "You have taken the approach of strength."