Musnad Ahmad 30:1697

Haytham narrated to us, Isma'il ibn 'Ayyash narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The expiation for gatherings is for the servant to say: 'Glory be to You, O Allah, and with Your praise, I seek Your forgiveness and I repent to You.'"