Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: This verse, "And do not recite your prayer aloud, nor recite it in a completely hushed voice, but seek a way between that" [Surah al-Isra: 110], was revealed because of those very (idolaters). He says: "Do not recite your prayer aloud, lest they run away from you, and do not recite it so quietly that the one who wants to hear it cannot hear it, those among them who wish to listen secretly, so that perhaps, having heard some of it, he may incline towards the call and benefit from it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11525