Hadithcore
Hadith #2103480

And in another report from Ibn Khuzaimah:

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

وَفِي أُخْرَى لِابْنِ خُزَيْمَةَ : { كَانُوا يُسِرُّونَ } .‏ [1]‏

References2 variants
Sunnah.com reference
Book 2, Hadith 165
Arabic/English book reference
Book 2, Hadith 0