(34) باب قَوْلِ اللَّهِ تَعَالَى {أَنْزَلَهُ بِعِلْمِهِ وَالْمَلاَئِكَةُيَشْهَدُونَ}
Narrated Ibn `Abbas:
(regarding the Verse):-- 'Neither say your prayer aloud, nor say it in a low tone.' (17.110) This Verse was revealed while Allah's Messenger (ﷺ) was hiding himself in Mecca, and when he raised his voice while reciting the Qur'an, the pagans would hear him and abuse the Qur'an and its Revealer and to the one who brought it. So Allah said:-- 'Neither say your prayer aloud, nor say it in a low tone.' (17.110) That is, 'Do not say your prayer so loudly that the pagans can hear you, nor say it in such a low tone that your companions do not hear you.' But seek a middle course between those (extremes), i.e., let your companions hear, but do not relate the Qur'an loudly, so that they may learn it from you.
Reference: Sahih al-Bukhari 7490
In-book reference: Book 97, Hadith 116
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 93, Hadith 582