Musnad Ahmad 27:710

Yahya narrated to us, from Ubaydullah, Nafi’ informed me, from Ibn Umar, who said: A man asked the Messenger of Allah, peace and blessings of Allah be upon him, about the night prayer. He said: "One of you should pray two by two. Then, if he fears that morning has arrived, he should pray one rak'ah, which will make his prayer odd for him."