(19) باب ائْتِمَامِ الْمَأْمُومِ بِالإِمَامِ
Jabir said:
The Messenger of Allah (ﷺ) led the prayer and Abu Bakr was behind him. When the Messenger of Allah (ﷺ) 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.
Reference: Sahih Muslim 413 b
In-book reference: Book 4, Hadith 91
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 825