Book 26: The Book on Salutations and Greetings (Kitab As-Salam)
Just as the face is the index to the mind, in the same way the words of greetings which are exchanged amongst people indicate the spiritual role of a society. The gestures, and the expressions of salutations are, therefore, the indicators of the ideals for which a particular society stands. The common practice with the pre-Islamic Arabs and thi modern West is to greet one another with Good Morning and Good Evening accompanied by a gesture of the hand or the nodding of head. These words have no spiritual and moral significance, but just a wish that one should find one's mornings and evenings happy. The words of greeting which Islam has exhorted its followers to use at the time of meeting are as-Salam-u-'Alaikum which have a deep spiritual and moral significance behind them Since the predominant feature of Islamic faith and practice is peace within and peace without, it has, therefore, been made a watchword which should be repeated on every occasion when two Muslims meet so that the utmost importance of peace should be constantly inculcated in the mind of every Muslim, and he should have full consciousness of the great reality that he lives in this world as a messenger of peace for mankind. peace amongst individuals and nations. -The second important feature of words of greeting in Islam is that they convey the message of peace not only to the person who is greeted but to the whole of the Muslim society as it is the plural pronoun that is used for the blessing of peace. (The pronoun'Alaikum has a evry wide range and all the Muslims come within its orbit.)
The two words as-Salam-u-'Alaikum may be called the irreducible minimum which are essential for greeting one another. The Muslims have been commanded to add to this expression some other words with a view to excelling one another in showering blessing upon the Muslims. The words as-Salam-u-'Alaikum are generally suffixed with these words Rahmat Allah wa Barakaatuh (mercy of Allah and His bleasings). These three words:" peace..., mercy" and" blessings of Allah" in fact form the sum and substance of Islam and the Muslims are asked to shower upon one another these benedictions. so that they may be able to live in this world and the Hereafter in perfect peace and tranquillity and blessings and mercy of Allah around him.
According to Imam Nawawl, greeting with the words of as-Salam-u-'Alaikum is a Sunnah, and responding to it is Wajib (obligatory), but this essentiality is of the nature of Kifaya, meaning thereby that if a few persons out of the whole assembly or group respond to the greeting it will absolve all of the responsibility of response but if no response comes forth from any quarter all are held responsible for it.