Musnad Ahmad 26:191

Yahya ibn Adam narrated to us, Sharik narrated to us, from Jami' ibn Abi Rashid, from Abu Wa'il, from Abdullah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to teach us the Tashahhud, just as he used to teach us the Surah from the Quran."