كتاب الصلاة
4  
The Book of Prayers
(31)
Chapter: Moderation When Reciting Qur'an In A Prayer When It Is To Be Recited Out Loud, And Making it Neither Too Loud Nor Too Soft, And When There Is The Fear Of Negative Consequences If It Is Recited Out Loud
(31)
باب التَّوَسُّطِ فِي الْقِرَاءَةِ فِي الصَّلاَةِ الْجَهْرِيَّةِ بَيْنَ الْجَهْرِ وَالإِسْرَارِ إِذَا خَافَ مِنَ الْجَهْرِ مَفْسَدَةً
Sahih Muslim 447 a

'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').
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا يَحْيَى بْنُ زَكَرِيَّاءَ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، فِي قَوْلِهِ عَزَّ وَجَلَّ ‏{‏ وَلاَ تَجْهَرْ بِصَلاَتِكَ وَلاَ تُخَافِتْ بِهَا‏}‏ قَالَتْ أُنْزِلَ هَذَا فِي الدُّعَاءِ ‏.‏
Reference : Sahih Muslim 447a
In-book reference : Book 4, Hadith 164
USC-MSA web (English) reference : Book 4, Hadith 898
  (deprecated numbering scheme)