If anyone sleeps before praying Witr or forgets it, let him pray it once he remembers it
Authentic hadithعن أبي سعيد الخدري رضي الله عنه مرفوعًا: «من نام عن وتره، أو نسيه، فَلْيُصَلِّه إذا ذكره».
Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported the Prophet (may Allah's peace and blessings be upon him) said: "If anyone sleeps before praying Witr or forgets it, let him pray it once he remembers it."
Explanation
A Muslim who sleeps until the morning without praying the Witr or forgets it, he should pray it after the dawn break, but not in the sense that it was missed and is made up for, rather in the sense that it is offered on its due time. The noble Hadīth shows that it is permissible for the Muslim who forgets or sleeps before praying the Witr to pray it even after the break of the second dawn, because he is excused in this case by the Shariah.
Source · Narrated by Ibn Majah - Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad
From the Hadith Encyclopedia (hadeethenc.com).