Musnad Ahmad 27:654

Muhammad ibn Abdullah ibn al-Zubayr—meaning Abu Ahmad al-Zubayri—narrated to us, he said: Abd al-Aziz—meaning Ibn Abi Rawwad—narrated to us, from Nafi, from Ibn Umar, who said: A man came to the Prophet, may God bless him and grant him peace, and asked him about the night prayer. He said: "The night prayer is two by two; you offer the salutation at every two rak'ahs. Then, if you fear the morning, pray one rak'ah; it will make what you have prayed before it an odd number (witr)."