Musnad Ahmad 27:1913

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, he said: Nafi' informed me that Ibn 'Umar used to say: "Whoever prays at night, let him make the end of his prayer Witr before the morning; that is how the Messenger of Allah (peace and blessings of Allah be upon him) used to command them."