Ya ayyuha allatheena amanoo la tarfaAAoo aswatakum fawqa sawti annabiyyi wala tajharoo (surah Hujurat: 2)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(49:2) Believers, do not raise your voices above the voice of the Prophet and when speaking to him do not speak aloud as you speak aloud to one another, *3 lest all your deeds are reduced to nothing without your even realising it. *4

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

*3) This is the etiquette that was taught to the people who sat among the audience of the Holy Prophet or came to visit him. Its intention was that the believers should treat 'the Holy Prophet with the highest respect and reverence when visiting him and talking to him. Nobody should raise his voice louder than his: the people should not be unmindful of the fact that they are addressing the Messenger of Allah, and not a common man, or a person of equal rank; therefore, there should be a marked difference between one's tone of conversation with the common people and one's tone of conversation with the Holy Prophet, and no one should talk to him in a voice louder than his.
Although this etiquette was taught for sitting in the Holy Prophet's assembly and its addressees were the people who were living in his time, the people of the later ages also should observe the same respect and reverence on the occasion when the Holy Prophet's name is mentioned, or a command of his is stated, or his sayings are explained. Besides, this verse also points out what attitude the people should adopt when talking to persons of a higher rank and status than themselves. A person's talking before the men of a higher rank in a way as he talks before his friends or the common men, is in fact a sign that he has no respect for them in his heart, and he does not recognize any difference between them and the common people.
*4) This shows what high position the person of the Holy Prophet occupies in Islam. No one beside the Holy Prophet, whatever his rank and status, has a position that unmannerly behavior towards him should deserve in the sight of AIlah the same punishment which is, in fact, the punishment for disbelief. In respect of ordinary people it is at the most a sort of rudeness, an uncivilized conduct, but in respect of the Holy Prophet a little lack of reverence is such a grave sin as can destroy all the services of one's lifetime. For the reverence of the Holy Prophet is indeed reverence of that God Who has sent him as His Messenger and lack of reverence for him amounts to lack of reverence to God Himself.