From Sahih Bukhari (Hadith 7547):
Narrated Ibn `Abbas:The Prophet (ﷺ) was hiding himself in Mecca and used to recite the (Qur'an) in a loud voice. When the pagans heard him they would abuse the Qur'an and the one who brought it, so Allah said to His Prophet: 'Neither say your prayer aloud, nor say it in a low tone
Reference: Bukhari 7547