Musnad Abu Ya'la al-Mawsili 108:54

Abu Bakr narrated to us, Husayn ibn Ali narrated to us, from Za'idah, from Ibn Aqil, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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)." Then 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 path of caution." And he said to Umar, "You have taken the path of strength."