Riyad us Saliheen: Hadith 833

Towards the end of his life, Messenger of Allah (ﷺ) would supplicate before leaving an assembly thus: "Subhanaka Allahumma wa bihamdika, ash-hadu an la ilaha illa Anta, astaghfiruka wa atubu ilaika (O Allah, You are free from every imperfection; all praise is for You. I testify that there is no true god except You, I ask Your forgiveness and turn to You in repentance).'' A man once said to him: "O Messenger of Allah! You have spoken such words as you have never uttered before.'' He said, "It is an expiation of that which goes on in the assembly.''

[Abu Dawud].