«كان إذا انصرف من صلاته استغفر ثلاثا ثم قال: اللهم أنت السلام ومنك السلام تباركت يا ذا الجلال والإكرام»
It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) finished his prayer, he would seek forgiveness three times, then say: 'O Allah, You are Peace and from You is peace; blessed are You, O Possessor of majesty and honour.'"
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4688