Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 825
Jabir said: The Messenger of Allah (may peace be upon him) led the prayer and Abu Bakr was behind him. When the Messenger of Allah (may peace be upon him) recited the takbir, Abu Bakr also recited (it) in order to make it audible to us. And the rest of the hadith is like one transmitted by Laith.