Musannaf Ibn Abi Shayba 3:4426

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Salim, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The night prayer is two by two. So if you fear the morning, then perform one rak'ah as witr."