When the Messenger of Allah ﷺ intended to conclude his prayer, he would seek forgiveness three times, then say, "O Allah, You are Peace, and from You comes peace. Blessed are You, Lord of Majesty and Honor."
حدثنا أبو المغيرة حدثنا الأوزاعي عن أبي عمار شداد عن أبي أسماء الرحبي
كان رسول الله ﷺ إذا أراد أن ينصرف من صلاته استغفر ثلاث مرات ثم قال اللهم أنت السلام ومنك السلام تباركت يا ذا الجلال والإكرام
Its isnad is sahih. Shaddad is Ibn Abdullah Dimashqi, a well-known trustworthy narrator whose hadith is accepted by the whole group. The hadith was narrated by Muslim, 1/414, in "Mosques," section on the recommendation of remembrance after prayer, by Nasai, 3/69, in "Inattentiveness," and by Ibn Majah, 1/298, no. 924.