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
← Previous Hadith "Aishah and Umm Salamah, the wives of the Prophet informed me that the Prophet would find that it was Fajr while he was Junub from (relations with) his wives, then he would perform Ghusl and fast."

Jami` at-Tirmidhi Book 8, Hadith 98 Next Hadith → 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