Musnad Ahmad 25:806

Yunus and Hujayn narrated to us, they said: Layth ibn Sa'd narrated to us, from Abu al-Zubayr, from Sa'id ibn Jubayr and Tawus, from Ibn Abbas, 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 Quran. He would say: "The blessed greetings, the prayers, and the pure words are for Allah. Peace be upon you — Hujayn said: 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 that Muhammad is the Messenger of Allah."