Sayyiduna Abdullah (bin Mas'ud) (may Allah be pleased with him) said: We used to say in the prayer: Peace be upon so-and-so, peace be upon so-and-so, and we would name (in greeting) whomever we knew among the angels and the prophets. Thereupon the Messenger of Allah (peace and blessings be upon him) said to us: "Indeed, Allah is Himself al-Salam (Peace)." Then he (peace and blessings be upon him) taught us the tashahhud, and we would learn it just as one of us would learn a surah of the Qur'an. He (peace and blessings be upon him) 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 / 9860