Oneness, Uniqueness of Allah (Tawheed)

Bukhari :: Book 9 :: Volume 93 :: Hadith 637

Narrated 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)