Musannaf Ibn Abi Shayba 3:4787

Humaid ibn Abd al-Rahman narrated to us, from his father, from Abu al-Zubayr, from Jabir, that the Messenger of Allah (may Allah bless him and grant him peace) led them in prayer, and Abu Bakr was behind him. The Prophet (may Allah bless him and grant him peace) would say the takbir, and Abu Bakr would say the takbir so that the people could hear.