Musannaf Ibn Abi Shayba 3:4417

Waki’ narrated to us, from Mis’ar, from Wabarah, who said: I asked Ibn Umar about a man who woke up and had not performed Witr. He said: "Have you considered if you slept through the Fajr [prayer] until the sun rose, would you not pray it?" As if he were saying: "He should perform Witr."