Musannaf Ibn Abi Shayba 3:116

Hafs narrated to us, from Abd al-Malik, who said: Sa’id ibn Jubayr used to say the takbir (Allahu Akbar) whenever he raised [his head] and whenever he bowed. He said: This was mentioned to Abu Ja’far, so he said: "He knows that it is the prayer of the Messenger of Allah, may Allah bless him and grant him peace." Sa’id said: "It is merely something with which a man adorns his prayer."