Salam Alaikum:-

phrase be to Allah

Allh(SWT)Say Al-Ahzab 56
Allh send his salat(Graces,Honours,Blessings,Mercy) on propht mhmmd(SAS),nd also his angles(ask allh to bless nd forgive him). O u who believe! send ur salat on(ask allh to bles) him (sas),nd (u should) great him wit d islamic way of greating(salutatn i.e. As-salamu ' Alikum).
Hence since allh(swt) nd angels show this extraordinary respect to propht mohammed(sas) every believer should do d same very regularly.it is interesting to note dat Allh(swt)has also great regard 4 d believa since he is involving them in an activity wch he nd his angels are engaged in. Hence whenever we hear d name of propht mhmmd(sas),we should do above activity ordered by d prophet (swt) Companions knew how to say salam to d prophet
i.e.
O propht, peace b upon u nd also allah's(swt)mercy nd blessings.
As mentioned in Bukhar nd narrated by kaab bin ujra(RU) One person asked d propht(sas),How to send salat to u, dear propht?The propht (sas) said as mentned in Bukhar nd Muslim.

Alahuma sooli ala mohmmad wahala ali mohmmad khama solaita ala ibrahim wahala ali ibrahim inaka amiidu majiidu,
Alahuma barkh ala mohmmad wahala ali mohmmad khama barakta ala ibrahima wahala ali ibrahima innka amiidu majiidu.)
There re other versions of dis too.
Allah(sas)also say in Al-Ahzab45,46

O propht(sas)! Verily, we hv sent u as witnes, nd a bearer of glad tiding,nd a warner, nd as one who invites to allh by his leave, nd as a lamp spreading,light(through ur instructions from d qur'an nd d sunnah d legal ways of d propht(sas).

Here five virtues of propht mohammad(sas) are desbribed.