Al-Hujuraat - 49:3

إِنَّ الَّذِينَ يَغُضُّونَ أَصْوَاتَهُمْ عِندَ رَسُولِ اللَّهِ أُوْلَئِكَ الَّذِينَ امْتَحَنَ اللَّهُ قُلُوبَهُمْ لِلتَّقْوَى لَهُم مَّغْفِرَةٌ وَأَجْرٌ عَظِيمٌ
[49:3]
Surely those who lower their voices before Allah's Messenger are they whose hearts Allah has proved for guarding (against evil); they shall have forgiveness and a great reward.
Those that lower their voices in the presence of Allah's Messenger,- their hearts has Allah tested for piety: for them is Forgiveness and a great Reward.
Lo! they who subdue their voices in the presence of the messenger of Allah, those are they whose hearts Allah hath proven unto righteousness. Theirs will be forgiveness and immense reward.