Musnad Ahmad 26:371

Mu'ammal narrated to us, Sufyan narrated to us, from Abu Ishaq, from al-Aswad, and Abu al-Ahwas, and Abu 'Ubaydah, from 'Abdullah, who said: The Prophet, peace and blessings of Allah be upon him, used to teach us the Tashahhud in prayer: "The greetings are for Allah, and the prayers and the good things. 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."