Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We used to pray with the Messenger of Allah (peace and blessings be upon him), and the people would say: Peace be upon Allah, peace be upon Jibril, peace be upon Mika'il, peace be upon the angels brought near. Thereupon the Prophet (peace be upon him) said: "Do not say: Peace be upon Allah, for Allah is Himself al-Salam (Peace)." The narrator said: Then he (peace and blessings be upon him) taught them the tashahhud and said: "Say: All verbal, physical and monetary acts of worship are for Allah alone. O Prophet, peace be upon you, and the mercy of Allah and His blessings. Peace be upon us and upon the righteous servants of Allah. I bear witness that there is no deity except Allah, and I bear witness that Muhammad is the servant of Allah and His Messenger."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9862