Silsilat al-Ahadith al-Sahihah

It is narrated from a man of the tribe of Banu Bayadah that the Messenger of Allah (peace and blessings be upon him) observed i'tikaf during one ten-day period of Ramadan and said: "When one of you is praying, he is whispering to his Lord, so do not recite the Qur'an aloud (in prayer), for in this way you cause discomfort to other believers."

It is narrated from Sayyiduna Abu Hurairah and Sayyidah Aisha (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) looked into a house and saw that people were praying and reciting aloud. He (peace and blessings be upon him) said to them: "Indeed, the one who is praying is whispering to his Lord, so he should pay attention to Whom he is whispering; therefore, (in prayer) when reciting the Noble Qur'an, you should not raise your voices over one another."

Attar bin Yasar narrates from an Ansari man of Banu Bayadah that the Prophet (peace and blessings be upon him) one day, while in the state of i'tikaf in the mosque, preached to the people, warned them, and encouraged them, then said: "Every worshipper is whispering to his Lord, so do not recite aloud over one another."