Sahih Bukhari | Volume 9 | Hadith Number 616

Narrated by Ibn 'Abbas
Regarding the explanation of the Verse: '(O Muhammad!) Neither say your prayer aloud, nor say it in a low tone.' (17:110) This Verse was revealed while Allah's Apostle was hiding himself at Mecca. At that time, when he led his companions in prayer, he used to raise his voice while reciting the Qur'an; and if the pagans heard him, they would abuse the Qur'an, its Revealer, and the one who brought it. So Allah said to His Prophet: "Neither say your prayer aloud. i.e., your recitation (of Qur'an) lest the pagans should hear (it) and abuse the Qur'an" nor say it in a low tone, "lest your voice should fail to reach your companions, "but follow a way between." (17:110)