Musnad Ahmad 27:1895

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Salim, from Ibn 'Umar, who said: The Prophet, peace and blessings of Allah be upon him, said: "The night prayer is two by two. So if you fear the morning, then perform one [rak'ah] as witr."