Musannaf Ibn Abi Shayba 3:627

Yahya ibn Adam narrated to us, from Sharik, from Jami' ibn Abi Rashid, from Abu Wa'il, from Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to teach us the Tashahhud just as he used to teach us the Surah from the Qur'an.