Musnad Ahmad 27:1034

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, I heard 'Uqbah ibn Hurayth, I heard Ibn 'Umar narrating from the Messenger of Allah, may Allah bless him and grant him peace, who said: "The night prayer is two by two. So if you see that the morning is about to overtake you, then perform one [rak'ah] as Witr." He said: It was said to Ibn 'Umar: "What is two by two?" He said: "You say the taslim after every two rak'ahs."