Arabic text
عن ابن عمر رضي الله عنهما أن رسول الله صلى الله عليه وسلم قال: «إذا دُعِيَ أحدكم إلى الوَلِيمَة فَلْيَأْتِهَا».
Translation
Ibn ‘Umar (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: “If one of you is invited to a wedding banquet, he should attend.”
Explanation
This Hadīth states that if a Muslim is invited to a wedding banquet, whether before, after, or at the same time of marriage consummation, then he must attend it to please his Muslim brother and share with him his joy. According to the majority of scholars, attending the wedding banquet is obligatory for the invitee, because, in other Hadīths, the Prophet (may Allah's peace and blessings be upon him) described the one who declines the invitation to a wedding banquet as disobedient to Allah, the Almighty, and His Messenger. This description can be assigned only to a person who fails to observe an obligation.
Source: HadeethEnc.com