Musnad Abu Ya'la al-Mawsili 153:191

Yahya ibn Ayyub narrated to us, Hushaym narrated to us, from Abd al-Rahman ibn Ishaq, from Muharib ibn Dithar, from Ibn Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to teach us the Tashahhud in the prayer just as the teacher teaches children.