When Islam came, Allah prescribed that the manner of greeting among Muslims should be Al-salamu ‘alaykum, and that this greeting should only be used among Muslims.

The meaning of “salam” (literally meaning “peace”) is harmlessness, safety and protection from evil and from faults. The name al-Salam is a Name of Allah, may He be exalted, so the meaning of the greeting of salam which is required among Muslims is, May the blessing of His Name descend upon you. The usage of the preposition ‘ala in ‘alaykum (upon you) indicates that the greeting is inclusive.

The most complete form of greeting is to say “As-Salamu `alaykum wa rahmatu-Allahi wa barakatuhu”, which means “Peace be upon you and the mercy of Allah and His blessings)”.

The Prophet ﷺ (peace be upon him) made spreading salam a part of faith.

A man asked the Messenger of Allah ﷺ (peace be upon him): What is the best thing in Islam? He said: Feeding others and giving the greeting of salam to those whom you know and those whom you do not know.

Sahih al-Bukhari, 12

Therefore, the Prophet ﷺ (peace be upon him) explained that giving salam spreads love and brotherhood. The Messenger of Allah ﷺ (peace be upon him) said:

You will not enter Paradise until you believe, and you will not believe until you love one another. Shall I not tell you about something which, if you do it, you will love one another? Spread salam amongst yourselves.

Sahih Muslim, 54

The Prophet ﷺ (peace be upon him) commanded us to return salams, and made it a right and a duty. He said:

The Muslim has five rights over his fellow Muslim: he should return his salams, visit him when he is sick, attend his funeral, accept his invitation, and pray for mercy for him [say Yarhamuk Allah] when he sneezes.

Sahih Muslim, 2162a

It is clear that it is obligatory to return salams, because by doing so a Muslim is giving you safety and you have to give him safety in return. It is as if he is saying to you, I am giving you safety and security, so you have to give him the same, so that he does not get suspicious or think that the one to whom he has given salam is betraying him or ignoring him.

Source: islamqa.info · islamqa.info