Musnad Ahmad 27:1912

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, Sulayman ibn Musa narrated to me, Nafi' narrated to us, that Ibn 'Umar used to say: "Whoever prays at night, let him make the end of his prayer Witr, for the Messenger of Allah (ﷺ) commanded that. When the dawn arrives, all night prayer and Witr have passed, for the Messenger of Allah (ﷺ) said: 'Perform Witr before the dawn.'"