The Prophet said: "When one of you is invited (to a meal) and he is fasting, then let him said: 'Indeed I am fasting.'"
Jami` at-Tirmidhi Book 8, Hadith 100
← Previous Hadith The Prophet said: "When one of you is invited to eat then let him respond, if he is fasting then let him pray." Meaning: supplicate.

Jami` at-Tirmidhi Book 8, Hadith 99 Next Hadith → The Prophet said: "A woman may not fast a day - other than in the month of Ramadan - while her husband is present, except with his permission."

Jami` at-Tirmidhi Book 8, Hadith 101