Sahih al-Bukhari 7526
Sahih al-Bukhari · Book 97: Oneness, Uniqueness of Allah (Tawheed) · “And whether you keep your talk secret or disclose it. Verily, He is the All-Knower of what is in the breasts (of men)….”
Narrated `Aisha:
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.
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.