After the Salam
أَسْتَغْفِرُ اللَّهَ، أَسْتَغْفِرُ اللَّهَ، أَسْتَغْفِرُ اللَّهَ، اللَّهُمَّ أَنْتَ السَّلَامُ وَمِنْكَ السَّلَامُ، تَبَارَكْتَ يَا ذَا الْجَلَالِ وَالْإِكْرَامِ
Astaghfirullah, astaghfirullah, astaghfirullah. Allahumma antas-salamu wa minkas-salam, tabarakta ya dhal-jalali wal-ikram.
I seek Allah's forgiveness, I seek Allah's forgiveness, I seek Allah's forgiveness. O Allah, You are Peace and from You comes peace. Blessed are You, Owner of majesty and honour.
What it means
The prayer ends with an apology rather than a congratulation. Even after standing before Allah you ask forgiveness three times for whatever was missing in it, then praise Him as the source of all peace.
When and how
Say it immediately after giving the salam, before any other dhikr or conversation.
Its virtue
This is what the Prophet ﷺ said when he finished his prayer, reported in Sahih Muslim 591.
Source
Muslim 591sahih