كتاب النكاح
16  
The Book of Marriage
(16)
Chapter: The command to accept invitations
(16)
باب الأَمْرِ بِإِجَابَةِ الدَّاعِي إِلَى دَعْوَةٍ ‏
Sahih Muslim 1429 c

Ibn Umar (Allah be pleased withthem) reported Allah's Apostle (ﷺ) having said this:

When any one of you is invited to a wedding feast, he should accept that.
حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا دُعِيَ أَحَدُكُمْ إِلَى وَلِيمَةِ عُرْسٍ فَلْيُجِبْ ‏"‏ ‏.‏
Reference : Sahih Muslim 1429c
In-book reference : Book 16, Hadith 114
USC-MSA web (English) reference : Book 8, Hadith 3339
  (deprecated numbering scheme)