Musannaf Ibn Abi Shayba 3:614

Ibn ‘Ulayyah narrated to us, from Khalid, from Abu al-Mutawakkil, who said: We asked Abu Sa’id about the Tashahhud. He said: "The greetings, the prayers, and the pure things are for Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us and upon the righteous servants of Allah. I bear witness that there is no god but Allah, and I bear witness that Muhammad is His servant and His messenger." Abu Sa’id said: "We used to not write anything except the Qur’an and the Tashahhud."