Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) used to teach us the tashahhud just as he would teach us a surah of the Qur'an: "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 true deity except Allah, and I bear witness that Muhammad is His servant and Messenger."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9879