Musannaf Ibn Abi Shayba 3:5707

Hussain bin Ali narrated to us, from Za'idah, from Abdullah bin Muhammad bin Aqil, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to Abu Bakr: "When do you perform the Witr?" He said: "At the beginning of the night, after the night prayer (al-'atamah), before I sleep."