The Book of Prayers (Kitab Al-Salat)

Muslim :: Book 4 : Hadith 897

Ibn 'Abbas reported: The word of (Allah) Great and Glorious: 'And utter not thy prayer loudly, nor be low in it" (xvii. 110) was revealed as the Messenger of Allah (may peace beupon him) was hiding himself in Mecca. When he led his Companions in prayer he raised his voice (while reciting the) Qur'an. And when the polytheists heard that, they reviled the Qur'an and Him Who revealed it and him who brought it. Upon this Allah, the Exalted, said to His Apostle (may peace be upon him): Utter not thy prayer so loudly that the polytheists may hear thy recitation and (recite it) not so low that it may be inaudible to your Companions. Make them hear the Qur'an, but do not recite it loudly and seek a (middle) way between these. Recite between loud and low tone.

Source materials are from the University of Southern California MSA site
Hadith eBooks converted from Imaan Star