Muwatta Imam Malik Book of Prayer, Hadith 30

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 ﷺ 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."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ مُحَمَّدِ بْنِ إِبْرَاهِيمَ بْنِ الْحَارِثِ التَّيْمِيِّ، عَنْ أَبِي حَازِمٍ التَّمَّارِ، عَنِ الْبَيَاضِيِّ، أَنَّ رَسُولَ اللَّهِ ﷺ خَرَجَ عَلَى النَّاسِ وَهُمْ يُصَلُّونَ وَقَدْ عَلَتْ أَصْوَاتُهُمْ بِالْقِرَاءَةِ فَقَالَ " إِنَّ الْمُصَلِّيَ يُنَاجِي رَبَّهُ فَلْيَنْظُرْ بِمَا يُنَاجِيهِ بِهِ وَلاَ يَجْهَرْ بَعْضُكُمْ عَلَى بَعْضٍ بِالْقُرْآنِ " .