Thauban (RA) said that Rasulullah (ﷺ) used to say after saying istighfar three times after the prayer:

اَللَّهُمَّ أَنْتَ السَّلَامُ وَمِنْكَ السَّلَامُ تَبَارَكْتَ يَا ذَا الْجَلَالِ وَالْإِكْرَامِ

Allaahumma 'Antas-Salaam wa minkas-salaam, tabaarakta yaa Dhal-Jalaali wal-'Ikraam

O Allah, You are Peace and from You comes peace. Blessed are You, O Owner of majesty and honor.

Reference: Muslim: 591