Musnad Ahmad 27:1709

Abu al-Mughirah narrated to us, Abdullah ibn al-Ala’—meaning Ibn Zabr—narrated to us, Salim ibn Abdullah narrated to me, from his father Abdullah ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked: "How is the night prayer?" He said: "Two by two. Then, if you fear the morning, perform one [rak'ah] as witr."