Abu Huraira (Allah be pleased with him) reported Allah’s Apostle (may peace be upon him) as saying: If any one of you is invited to a meal when he is fasting, he should say: “I am fasting.”

Abu Huraira (Allah be pleased with him) reported Allah’s Apostle (may peace be upon him) as saying: If any one of you is invited to a meal when he is fasting, he should say: “I am fasting.”
Source: HOTD