Sahih Bukhari | Volume 9 | Hadith Number 637

Narrated by Ibn 'Abbas
The Prophet 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.' (17:110)