Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "The expiation for a gathering is that the servant says: 'Glory be to You, O Allah, and with Your praise; I bear witness that there is no true god except Allah; I seek Your forgiveness and I turn to You in repentance.'"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10295