It is narrated from Sayyiduna al-Mughirah bin Shu'bah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him), after ending with the salam at the close of the prayer, used to say: "There is no true god except Allah, He is alone, He has no partner; His is the dominion and to Him belongs all praise, and He has power over all things. O Allah, none can withhold what You give, and none can give what You withhold, and the greatness of one who has greatness avails him nothing against You."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18042