Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 786
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.