Sahih al-Bukhari 4723
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · "...And offer your Salāt (prayer) neither aloud nor in a low voice..." (V.17:110)
Narrated Aisha:
حَدَّثَنِي طَلْقُ بْنُ غَنَّامٍ ، حَدَّثَنَا زَائِدَةُ ، عَنْ هِشَامٍ ، عَنْ أَبِيهِ ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ أُنْزِلَ ذَلِكَ فِي الدُّعَاءِ.
The (above) verse was revealed in connection with the invocations.