Hadith 737
Abu Hurairah (May Allah be pleased with him) reported:
عن أبى هريرة رضى الله عنه قال: قال رسول الله صلى الله عليه وسلم : "إذا دعي أحدكم ، فليجب، فإن كان صائما فليصل، وإن كان مفطراً فليطعم" .((رواه مسلم))
The Messenger of Allah (ﷺ) said, "When any of you is invited to a meal, he should accept the invitation. If he is observing Saum (fasting), he should supplicate for the betterment of the host and if he is not fasting, he should eat." [Muslim].
In plain words
When invited to a meal, accept the invitation. If you are fasting, pray for blessing on your host instead of eating, and if not, then eat. Either way, honor the one who invited you.
Reference : Riyad as-Salihin 737 In-book reference : Book 2, Hadith 11