There are some expressions which a man utters three times when he gets up from an assembly he will be forgiven for what happened in the assembly; and no one utters them in an assembly held for a noble cause or for remembrance of Allah but that is stamped with them just as a document is stamped with a signet-ring. These expressions are: Glory be to Thee, O Allah, and I begin with praise of Thee, there is no god but thou; I ask Thy pardon, and return to Thee in repentance.  

أَنَّهُ قَالَ كَلِمَاتٌ لاَ يَتَكَلَّمُ بِهِنَّ أَحَدٌ فِي مَجْلِسِهِ عِنْدَ قِيَامِهِ ثَلاَثَ مَرَّاتٍ إِلاَّ كُفِّرَ بِهِنَّ عَنْهُ وَلاَ يَقُولُهُنَّ فِي مَجْلِسِ خَيْرٍ وَمَجْلِسِ ذِكْرٍ إِلاَّ خُتِمَ لَهُ بِهِنَّ عَلَيْهِ كَمَا يُخْتَمُ بِالْخَاتَمِ عَلَى الصَّحِيفَةِ سُبْحَانَكَ اللَّهُمَّ وَبِحَمْدِكَ لاَ إِلَهَ إِلاَّ أَنْتَ أَسْتَغْفِرُكَ وَأَتُوبُ إِلَيْكَ  

A similar tradition has also been transmitted by Abu Hurairah from the Prophet ﷺ through a different chain of narrators.  

عَنِ النَّبِيِّ ﷺ مِثْلَهُ  

When the Messenger of Allah ﷺ intended to get up from the assembly he used to say in the last. Glory be to Thee. O Allah, and I begin with praise of Thee, I testify that there is no god but Thou; I ask Thy pardon, and return to Thee in repentance. The man asked: Messenger of Allah! you utter the words now which you did not do in the past? He replied: (This is an) atonement for what takes place in the assembly.  

كَانَ رَسُولُ اللَّهِ ﷺ يَقُولُ بِأَخَرَةٍ إِذَا أَرَادَ أَنْ يَقُومَ مِنَ الْمَجْلِسِ سُبْحَانَكَ اللَّهُمَّ وَبِحَمْدِكَ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ أَنْتَ أَسْتَغْفِرُكَ وَأَتُوبُ إِلَيْكَ فَقَالَ رَجُلٌ يَا رَسُولَ اللَّهِ إِنَّكَ لَتَقُولُ قَوْلاً مَا كُنْتَ تَقُولُهُ فِيمَا مَضَى قَالَ كَفَّارَةٌ لِمَا يَكُونُ فِي الْمَجْلِسِ