Maliks Muwatta Book 3, Hadith Number 30.

Section : Behaviour in the Recitation.

Yahya related to me from Malik from Yahya ibn Said from Muhammad ibn Ibrahim ibn al Harith at-Taymi from Abu Hazim at-Tammar from al Bayadi that the Messenger of Allah, may Allah bless him and grant him peace, came out to the people while they were praying and their voices were raised in the recitation. He said, “When you pray you are talking confidentially to your Lord. So look to what you confide to Him, and do not say the Qur’an out loud so that others hear it.”