Musannaf Ibn Abi Shaybah 7353 — What the Prophet (peace and blessings)

Sayyiduna Jabir (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) led the people in prayer, and Sayyiduna Abu Bakr (may Allah be pleased with him) was behind him. Whenever the Prophet (peace and blessings be upon him) said the takbir, Sayyiduna Abu Bakr would say the takbir aloud to convey the sound of the takbir to the people.