Musnad Ahmad 27:538

'Abd al-A'la ibn 'Abd al-A'la narrated to us, from Khalid, from 'Abdullah ibn Shaqiq, from Ibn 'Umar, that the Prophet (peace and blessings of Allah be upon him) said: "The night prayer is two by two. So if you fear the dawn, then perform one [rak'ah] as witr."