Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that 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, and he would say: "There is no prayer without the tashahhud."
With its sound supporting narrations (shawahid), this hadith gains some degree of strength.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3963