Sahih Muslim Book 4, Hadith Number 786.

Chapter : The one led in prayer is forbidden to recite loudly behind the Imam.

Anas reported: I observed prayer along with the Messenger of Allah (may peace be upon him) and with Abu Bakr, Umar and Uthman (may Allah be pleased with all of them), but I never heard any one of them reciting Bismillah-ir-Rahman-ir-Rahim loudly.