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كتاب التوحيد
97  
Oneness, Uniqueness of Allah (Tawheed)
(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)….”
(44)
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)