Musannaf Ibn Abi Shayba 3:104

Yahya ibn Sa'id narrated to us, from Malik ibn Anas, from Wahb ibn Kaysan, who said: Jabir ibn 'Abd Allah used to teach us the *takbir* (saying "Allahu Akbar") in the prayer. We would say *takbir* when we lowered (ourselves) and when we raised (ourselves).