Sunan Ibn Majah
1188
Sahih+2

It was narrated that Abu Sa’eed said:“The Messenger of Allah (ﷺ) said: ‘Whoever sleeps and misses Witr, or forgets it, let him pray it when morning comes, or when he remembers.’”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi