Musnad Ahmad 2:74

Hashim narrated to us, Abu Bishr informed us, from Sa'id ibn Jubayr, from Ibn Abbas, who said: This verse was revealed while the Messenger of Allah (peace and blessings of Allah be upon him) was in hiding in Makkah: "And do not recite [too] loudly in your prayer, nor recite too quietly" [Al-Isra: 110]. He said: When he used to pray with his companions, he would raise his voice with the Qur'an. He said: When the polytheists heard that, they would revile the Qur'an, the One who revealed it, and the one who brought it. So Allah, the Almighty and Majestic, said to His Prophet (peace and blessings of Allah be upon him): "And do not recite [too] loudly in your prayer," meaning your recitation, lest the polytheists hear it and revile the Qur'an. "Nor recite too quietly" from your companions, so that you do not let them hear the Qur'an until they learn it from you. "But seek between that a way."