Musnad Ahmad 27:1716

Ya'qub narrated to us, Ibn Akhi Ibn Shihab narrated to us, from his uncle, Humayd ibn Abd al-Rahman ibn Awf informed me, that Abdullah ibn Umar informed him, that a man asked the Messenger of Allah, peace and blessings of Allah be upon him, about the night prayer. The Messenger of Allah, peace and blessings of Allah be upon him, said: "The night prayer is two by two. So if you fear the morning, then perform one [rak'ah] as witr."