Musannaf Ibn Abi Shayba 3:4423

Hushaym narrated to us, he said: Yunus informed us from al-Hasan, and al-Mughirah from Ibrahim, and ‘Abd al-Malik from ‘Ata’, that they said: If you have not done [it] and the dawn has broken, then perform the Witr as long as you have not prayed the morning prayer.