Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever enters without an invitation enters as a raider and leaves as a thief."
(b) Al-Bahrani added to it: "The wedding feast (walimah) is a right; whoever does not accept the invitation to the walimah has disobeyed Allah and His Messenger."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14546