Sayyiduna Ibn Abbas (may Allah be pleased with them both) says, regarding the saying of Allah Almighty: "And do not recite your prayer aloud, nor recite it too quietly, but seek a way between that" [al-Isra: 110]: When the Messenger of Allah (peace and blessings be upon him) led the people in prayer, he would recite the Qur'an aloud, and when the polytheists heard it, they would revile the Qur'an and the one who brought it. So Allah Almighty revealed to him (peace and blessings be upon him): "And do not recite your prayer aloud" [al-Isra: 110], lest the polytheists hear it and revile it; "nor recite it too quietly" [al-Isra: 110], meaning let your companions hear it; "but seek a way between that" [al-Isra: 110].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12401