Musnad Ahmad 27:1548

Hashim narrated to us, Al-Layth narrated to us, Nafi’ narrated to me, from Abdullah, from the Messenger of Allah (peace and blessings of Allah be upon him) that he said: "The night prayer is two by two. If you fear the morning, then pray one [rak'ah] as Witr, and make the end of your prayer Witr."