Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3348
Abu Haraira (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: If any one of you is invited, he should accept (the invitation). In case he is fasting, he should pray (in order to bless the inmates of the house), and if he is not fasting he should eat.