Musannaf Ibn Abi Shayba 3:613

Al-Fadl ibn Dukayn narrated to us, from Sufyan, from Zayd al-Ammi, from Abu al-Siddiq al-Naji, from Ibn Umar, that Abu Bakr used to teach them the Tashahhud upon the minbar just as children are taught in the school: "The greetings, prayers, and good 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."