Musannaf Ibn Abi Shayba 21:206

Ibn Fudayl narrated to us, from Mujahid, from Abdullah ibn Umar, who said: Whoever says when he stands up from his gathering, "Glory be to You, O Allah, and with Your praise; I bear witness that there is no god but You; I seek Your forgiveness and I repent to You," Allah will suffice for him [the expiation of] every sin in that gathering.