Musnad Abu Ya'la al-Mawsili 153:80

Amr narrated to us, Sufyan narrated to us, from al-Zuhri, from Salim, from his father, from the Prophet (may Allah bless him and grant him peace), who said: "The night prayer is two by two. So if you fear the morning, then perform one rak'ah as witr, which will make witr for you what has passed."