Jami` at-Tirmidhi 469
جامع الترمذي#469SahihIbn Umar narrated that :Allah's Messenger said: "When Fajr begins, then every Salat of the night and Al-Witr have gone, so perform Al-Witr before Fajr begins
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 3:17