Jami` at-Tirmidhi 730
جامع الترمذي#730Da'ifHafsah narrated that:the Prophet said: "Whoever did not decide to fast before Fajr then there is no fast for him
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 ZaiDaif
In-book reference: 8:49