Jami` at-Tirmidhi 781
جامع الترمذي#781SahihAbu Hurairah narrated that :The Prophet said: "When one of you is invited (to a meal) and he is fasting, then let him said: 'Indeed I am fasting
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
Bashar Awad MaaroufHasan Sahih
Zubair Ali ZaiSahih Muslim
In-book reference: 8:100