Reference : Hisn al-Muslim 66

أَسْـتَغْفِرُ الله (ثَلاثاً) 
اللّهُـمَّ أَنْـتَ السَّلامُ
وَمِـنْكَ السَّلام
تَبارَكْتَ يا ذا الجَـلالِ وَالإِكْـرام
Astaghfirullāh (three times) 
Allāhumma antas-salām, 
wa minkas-salām, 
tabārakta yā dhal-Jalāli wal-'Ikrām.

I seek the forgiveness of Allah (three times). 
O Allah, You are Peace and from You comes peace. 
Blessed are You, O Owner of majesty and honor.

Reference:
Muslim 1/414.

Chapter: After salam