Qur'an, Al-Hujurat, Surah 49:2-3
Believers, do not raise your voices above my voice, the voice of the prophet. Do not shout at me, as you often shout at one another. If you raise your voices and shout, you will not hear the truth, and so your labours will come to nothing. Those who speak softly in the presence of God's apostle, and those whom God has made pious, will be forgiven their sins, and they will receive a rich reward.