Silsilah as-Sahihah 521 — Prayer is whispering to Allah; how loud

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."