Warrad, the freed slave of Sayyiduna al-Mughirah (may Allah be pleased with him), relates that Sayyiduna al-Mughirah (may Allah be pleased with him) wrote to Mu'awiyah (may Allah be pleased with him) - it was Warrad himself who wrote that letter -: I heard the Messenger of Allah (peace and blessings be upon him) say, when ending the prayer with the salam: "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 / 18048