Hadith 165

Bulugh al-Maram

Book 2: The Book of Prayer
وَفِي أُخْرَى لِابْنِ خُزَيْمَةَ : { كَانُوا يُسِرُّونَ } .‏ 1‏
And in another report from Ibn Khuzaimah:

"They used to recite Bismillahir-Rahmanir-Rahim silently."