From Sahih Bukhari (Hadith 4722):
Narrated Ibn `Abbas:(regarding): '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. When he prayed with his companions, he used to raise his voice with the recitation of Qur'an, and if the pagans happened to hear him, they would abuse the Qur'an, the One who revealed it and the one who brought it. Therefore Allah said to His Prophet : 'Neither say your prayer aloud.' (17.110) i.e. do not recite aloud lest the pagans should hear you, but follow a way between
Reference: Bukhari 4722