Musnad Abu Ya'la al-Mawsili 111:11

Abu Ya'la informed us, he said: It was read to Bishr, Abu Yusuf informed you, from Ibn Abi Layla, from Nafi', from Ibn 'Umar, from the Messenger of Allah (may Allah bless him and grant him peace), that he said: "The night prayer is two by two. So if you fear the morning, then perform one [rak'ah] as Witr. Indeed, Allah is Witr (One/Odd) and He loves the Witr, because He is One."