Musnad Ahmad 27:1100

'Abd al-Samad narrated to us, Harun ibn Ibrahim al-Ahwazi narrated to us, Muhammad ibn Sirin narrated to us, from Ibn 'Umar, that the Prophet, peace and blessings of Allah be upon him, said: "The Maghrib prayer is the Witr (odd-numbered prayer) of the day, so make the night prayer Witr. The night prayer is two by two, and the Witr is one rak'ah from the end of the night."