Tirmidhi

1:
The instruction to say "I am fasting" is as an excuse for not accepting the invitation.
Although it is better to conceal voluntary (nafl) acts,
here the command to disclose it is so that the host does not harbor any misunderstanding or resentment in his heart against the invited person.

Hadith Commentary:
Benefits and Issues:
From the hadith of Abu Hurairah (radi Allahu anhu), it is established that the period in which the questioner asked the Prophet (sallallahu alayhi wa sallam) about praying in a single garment was a time of extreme poverty and need, and not every person had the means to wear two garments. Therefore, the Shariah did not specify a set number of garments for prayer.
A person should pray in whatever capacity and means he possesses, or in however many clothes he usually wears, but it is necessary to cover the ‘awrah (private parts that must be concealed).

Benefits and Issues:
Participating in a food invitation is superior.
However, if one excuses himself and informs that he is fasting, that is also permissible.
Or, it also means that if one informs the people present in the gathering about his fast, there is no fault in that.

Benefits and Issues:
➊ When a fasting person is invited to a meal, it is permissible for him to break his fast and accept the invitation, joining in the meal. It is also permissible for him to excuse himself from eating.

➋ Informing the host that one is fasting does not fall under ostentation (riya), because the purpose is not to announce one’s piety, but rather to express one’s excuse.

➌ This ruling applies to voluntary (nafl) fasts; it is not permissible to break an obligatory (fard) fast except in the case where there is a valid excuse such as travel or illness, due to which breaking the fast becomes permissible according to the Shari‘ah.