Musnad Ahmad 27:892

Abu Ahmad al-Zubayri narrated to us, Abd al-Aziz—meaning Ibn Abi Rawwad—narrated to us, from Nafi, from Ibn Umar, who said: A man came to the Prophet, may Allah bless him and grant him peace, and asked him about the night prayer. He said: "The night prayer is two by two; you say the taslim after every two rak'ahs. If you fear the morning, then pray one rak'ah; it will make witr for you what came before it."