Musnad Ahmad 27:1961

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