Translation
(97) Oneness, Uniqueness of Allah (Tawheed)
كتاب التوحيد
(44) Chapter: “And whether you keep your talk secret or disclose it. Verily, He is the All-Knower of what is in the breasts (of men)….”
Sahih al-Bukhari 7526 

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.

حَدَّثَنَا عُبَيْدُ بْنُ إِسْمَاعِيلَ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ نَزَلَتْ هَذِهِ الآيَةُ ‏{‏وَلاَ تَجْهَرْ بِصَلاَتِكَ وَلاَ تُخَافِتْ بِهَا‏}‏ فِي الدُّعَاءِ‏.‏
Reference : Sahih al-Bukhari 7526
In-book reference : Book 97, Hadith 151
USC-MSA web (English) reference : Vol. 9, Book 93, Hadith 617
  (deprecated numbering scheme)