When he finished the prayer, the Messenger of Allah ﷺ would pray for forgiveness three times and say: 'Allahumma anta asalam, wa minka as-salam tabarakta ya dhal-jalali wal-ikram (O Allah, You are the source of eace (or the One free from all faults) and from You comes peace, blessed are You, O Possessor of Majesty and Honor)." (Using translation from Nasāʾī 1337)  

أَنَّ رَسُولَ اللهِ ﷺ كَانَ إِذَا انْصَرَفَ مِنْ صَلَاتِهِ اسْتَغْفَرَ ثَلَاثًا قَالَ «اللهُمَّ أَنْتَ السَّلَامُ وَمِنْكَ السَّلَامُ تَبَارَكْتَ يَا ذَا الْجِلَالِ وَالْإِكْرَامِ»