All perfect praise be to Allah The Lord of The Worlds; and may his blessings and peace be upon our Prophet Mohammad and upon all his Family and Companions.
Islam has enjoined Muslims to promote greeting(by saying As-salamu 'alaikum to one another)"amongst themselves in order to strengthen ties of love and sympathy. Abu Hurairah (May Allah be pleased with him) reported: "Messenger of Allah (PBUH) said, "By Him in Whose Hand my soul is! You will not enter Jannah until you believe, and you shall not believe until you love one another. May I inform you of something, if you do, you love each other. Promote greeting amongst you (by saying As-salamu 'alaikum to one another)". [Muslim].
Replying the greeting is an obligation, for Allah the Almighty Says {what means}: “When ye are greeted with a greeting, greet ye with a better than it or return it. “ {An-Nisa`/86}. If one person greeted a group of persons, it is sufficient that one of them greets him back because in this situation it is a collective duty. Al-Imam Zakaria Al-Ansari(May Allah be pleased with him) said: “ Returning the Salam is an individual duty if the greeted is one responsible person, and a collective duty if the greeted were a group of persons.” {Assna Al-Matalib 4/183}.
In principle, although greeting others with Salam is recommended, it is discouraged to do so if the greeted is in one of the following situations: performing prayer, calling for prayer (Azaan) and Iqama(starting the prayer), reciting the Quran, offering Talbiya, having a lesson, listening to Friday sermon, sleeping, or being sleepy, chewing food, urinating, or bathing. Although Islam discourages a Muslim from greeting others while they are praying, offering Azan, or Iqama, supplicating Allah, reciting the Quran, or offering Talbiya, having a lesson, and during Friday sermon, Muslim jurists have differed in this regard.
The Hanafites state that replying the Salam isn`t an obligation because their adopted maxim in this regard reads as follows: “ Every situation where greeting with Salam isn`t recommended, greeting back isn`t considered an obligation.” {Hashyaat Ibn Abdeen}.
Whereas, the Shafites state that it is an obligation. Al-Imam Ar-Ramli (May Allah bless his soul) said: “ If a person enters a mosque during Friday sermon and gives Salam, it is an obligation on those listening to the sermon to greet back although it is disliked for the one who entered the mosque to have given Salam in the first place. However, it isn`t an obligation on the one answering the call of nature to reply the Salaam because doing so is considered an act of stupidity on his part, contrary to the persons in other situations.” {Nehayat Al-Mohtajj 2/321}.
