Surah Al-Hujurat (The Private Quarters), Verse 2

یٰۤاَیُّهَا الَّذِیْنَ اٰمَنُوْا لَا تَرْفَعُوْۤا اَصْوَاتَكُمْ فَوْقَ صَوْتِ النَّبِیِّ وَلَا تَجْهَرُوْا لَهٗ بِالْقَوْلِ كَجَهْرِ بَعْضِكُمْ لِبَعْضٍ اَنْ تَحْبَطَ اَعْمَالُكُمْ وَاَنْتُمْ لَا تَشْعُرُوْنَ ۟

Yaa ayyuhal lazeena aamanoo laa tarfa'ooo aswaatakum fawqa sawtin Nabiyi wa laa tajharoo lahoo bilqawli kajahri ba'dikum liba 'din an tahbata a 'maalukum wa antum laa tash'uroon

“O ye who believe! Raise not your voices above the voice of the Prophet, nor speak aloud to him in talk, as ye may speak aloud to one another, lest your deeds become vain and ye perceive not.”

Quran 49:2 (Yusuf Ali Translation)