Musnad Abu Ya'la al-Mawsili 153:206

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Sulayman al-Taymi informed us, from Tawus, from Ibn Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "The night prayer is two by two. So if you fear the morning, then perform one [rak'ah] as witr."