(44) باب قَوْلِ اللَّهِ تَعَالَى {وَأَسِرُّوا قَوْلَكُمْ أَوِ اجْهَرُوابِهِ إِنَّهُ عَلِيمٌ بِذَاتِ الصُّدُورِ أَلاَ يَعْلَمُ مَنْ خَلَقَ وَهُوَ اللَّطِيفُ الْخَبِيرُ}
Narrated `Aisha:
The Verse:-- '(O Muhammad!) Neither say your prayer aloud nor say it in a low tone.' (17.110) was revealed in connection with the invocations.
Reference: Sahih al-Bukhari 7526
In-book reference: Book 97, Hadith 151
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 93, Hadith 617