Musannaf Ibn Abi Shayba 3:625

Yahya ibn Adam narrated to us, he said: Abd al-Rahman ibn Humayd narrated to us, he said: Abu al-Zubayr narrated to me, from Tawus, from Ibn Abbas, he 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.