Hadith 2:161
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 ِ }