Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We did not know what to say in the prayer; we used to say: Peace be upon Allah, peace be upon Jibril, peace be upon Mika'il. Then the Messenger of Allah (peace and blessings be upon him) taught us the tashahhud and said: "Indeed, Allah Himself is al-Salam. When you sit after two rak'ahs, say: All verbal, physical and financial acts of worship are for Allah alone. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon the righteous servants of Allah. (Abu Wa'il said: In the hadith of Sayyiduna Abdullah (may Allah be pleased with him) from the Prophet (peace and blessings be upon him): When you say it, it reaches every righteous servant, whether in the heaven or on the earth. Abu Ishaq said: In the hadith of Sayyiduna Abdullah (may Allah be pleased with him): When you say it, it reaches every angel brought near, or prophet sent as a messenger, or righteous servant.) I bear witness that there is no god but Allah, and I bear witness that Muhammad is the servant of Allah and His Messenger."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9852