In another narration from Ahmad, an-Nasa'i and Ibn Khuzaimah:

"They never used to recite Bismillahir-Rahmanir-Rahim aloud."

وَفِي رِوَايَةٍ لِأَحْمَدَ , وَالنَّسَائِيِّ وَابْنِ خُزَيْمَةَ : { لَا يَجْهَرُونَ ‏بِبِسْمِ اَللَّهِ اَلرَّحْمَنِ اَلرَّحِيم ِ } [1]‏