The Book of Prayer

Bulugh al-Maram #341

كتاب الصلاة

وَفِي أُخْرَى لِابْنِ خُزَيْمَةَ: «كَانُوا يُسِرُّونَ».

And in another report from Ibn Khuzaimah:

“They used to recite Bismillahir-Rahmanir-Rahim silently.”