Yahya related to me from Malik from Yahya ibn Said from Muhammadibn Ibrahim ibn al Harith at-Taymi from Abu Hazim at-Tammar from alBayadi that the Messenger of Allah, may Allah bless him and grant himpeace, came out to the people while they were praying and their voiceswere raised in the recitation. He said, "When you pray you are talkingconfidentially to your Lord. So look to what you confide to Him, anddo not say the Qur'an out loud so that others hear it."
App reference: Book 3, Hadith 30
USC-MSA web (English) reference: Book 3, Hadith 30
Arabic reference: Book 3, Hadith 177