(31) باب التَّوَسُّطِ فِي الْقِرَاءَةِ فِي الصَّلاَةِ الْجَهْرِيَّةِ بَيْنَ الْجَهْرِ وَالإِسْرَارِ إِذَا خَافَ مِنَ الْجَهْرِ مَفْسَدَةً
'A'isha reported that so far as these words of (Allah) Glorious and High are concerned:
" And utter not thy prayer loudly, not be low in it" (xvii. 110) relate to supplication (du'a').
Reference: Sahih Muslim 447 a
In-book reference: Book 4, Hadith 164
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 898