Musnad Ahmad 27:1979

Muhammad ibn Ubayd narrated to us, al-A’mash narrated to us, from ‘Atiyyah ibn Sa’d, from Ibn ‘Umar, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "The prayer of the night is two by two. So if you fear the morning, then [pray] one. Indeed, Allah the Exalted is Witr (Odd/One) and He loves the Witr."