Arabic text
عن أبي هريرة رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «إذا دُعِيَ أحدكم فليُجب، فإن كان صائما فليُصَلِّ، وإن كان مُفْطِرا فليَطْعَمْ».
Translation
Abu Hurayrah (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If anyone of you is invited, let him answer the invitation. If he is fasting, let him supplicate (for the inviter); if he is not fasting, let him eat."
Explanation
This Hadīth shows that if a Muslim invites his brother to a marriage banquet, he should answer the invitation so as to please his brother and attend such a happy event. However, if one is observing an obligatory fast, in which he makes up for missed days, or fasts a day that he vowed to fast, then he should attend the party without eating. Still, he should invoke Allah's goodness and blessings upon his inviter. If he is observing a supererogatory fast, he may either break his fast and eat with the inviter or continue his fast. However, he should tell the inviter whether or not he is fasting so as to avoid putting him in an awkward situation. In any case, answering the invitation is confirmed regardless of whether one is fasting or not.
Source: HadeethEnc.com