Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) used to teach us the tashahhud just as he would teach us a surah of the Qur'an, and he would say: "Learn it, for there is no prayer without the tashahhud."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9886