The freed servant of the Messenger of Allah ﷺ reported that whenever the Messenger of Allah ﷺ finished his prayer, he would seek forgiveness three times and say, "O Allah, You are Peace, and from You comes peace. Blessed are You, Possessor of Majesty and Honor."
أخبرنا محمود بن خالد قال حدثنا الوليد عن أبي عمرو الأوزاعي قال حدثني شداد أبو عمار أن أبا أسماء الرحبي حدثه أنه
مولى رسول الله ﷺ يحدث أن رسول الله ﷺ كان إذا انصرف من صلاته استغفر ثلاثا وقال اللهم أنت السلام ومنك السلام تباركت يا ذا الجلال والإكرام
Muslim narrated it in "the Book of Mosques," chapter "The Recommendation of Remembrance After Prayer and a Description of It," hadith no. 591, from the hadith of Walid bin Muslim through this route, and it is also in Al-Kubra, hadith no. 1260.