Musannaf Ibn Abi Shayba 3:630

Ibn Fudayl narrated to us, from al-A’mash, from Ibrahim, from al-Aswad, who said: ‘Abdullah used to teach us the Tashahhud in prayer just as he used to teach us a Surah from the Qur’an. He would correct our ‘alif’ and our ‘waw’.