يَا أَيُّهَا الَّذِينَ آمَنُوا لَا تَدْخُلُوا بُيُوتًا غَيْرَ بُيُوتِكُمْ حَتَّى تَسْتَأْنِسُوا وَتُسَلِّمُوا عَلَى أَهْلِهَا ذَلِكُمْ خَيْرٌ لَّكُمْ لَعَلَّكُمْ تَذَكَّرُونَ
O you who believe! Enter not houses other than your own, until you have asked permission and greeted those there in".
(Surah An-Noor: Verse 27)
فَإِذَا دَخَلْتُم بُيُوتًا فَسَلِّمُوا عَلَى أَنفُسِكُمْ تَحِيَّةً مِّنْ عِندِ اللَّهِ مُبَارَكَةً طَيِّبَةً
...But when you enter the houses, greet one another with a greeting from Allah: a greeting blessed and good...
(Surah An-Noor: verse 61)
وَإِذَا حُيِّيْتُم بِتَحِيَّةٍ فَحَيُّواْ بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَا
"When you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally..."
(Surah An-Nisa: verse 86)
When entering or leaving the house, one must acknowledge those inside by using the greetings of Muslims and the motto of Islam;
"May the peace and blessings of Allah be upon you"
The Prophet (صلى الله عليه وسلم) said:
"A Muslim owes another Muslim six duties. To greet him when he meets him, to accept when he gives him an invitation, to give him good counsel when he asks him for advice, to pray for mercy for him when he sneezes and says "Praise be to Allah" , to visit him when he is ill, and follow him (attend his funeral) when he dies".
We should not take preference of alternative greeting forms such as "Good Morning", "Good Evening", "Hello" or an arabic greeting such as sabah al khair/ marhaba/ ahlan wasahlan etc. in the place of this greeting. This greeting is the sign of Islam and the phrase the Messenger of Allah (صلى الله عليه وسلم) reccommended and used. The Messenger of Allah (صلى الله عليه وسلم) said to Anas (رضى الله عنه) :
"My son, greet you family when you enter your home, for that is a blessing for you and your family"
The Messenger of Allah (صلى الله عليه وسلم) also said:
"When one of you comes to an assembly, he should give the greeting. If he leaves he should give the greeting. Neither greeting is more deserving than the other."
(Al Adab Al Mufrad)
The greeting is deemed by many scholars to be wajib or sunnat ul mu'aqada whilst the reply is obligatory. So one is sinful for not giving salam. However when we make the intention to give greetings of salam we refer to 3; the person we meet, and the 2 angels. So the greeting of salam is of much greater value than "hi", "hello" etc, we have the intention of the dua'a which is multiple depending on the length of our salam and the degree to which we extend it. No matter whether our salam is given a reply by the person, the angels answer, so even when we enter an empty house, one should remember to make the greeting of salam.
So for example Asalamu-alaykum is plural, whilst Asalamu-alayka is the singular of the greeting. So in turn we should be generous with our greetings, we should when greeted reply with a greeting, a dua'a of equal or greater extent to gain extra reward.
In turn one must not be miserly with their reply. Abdullah Ibn 'Amr Ibn al-'As (رضى الله عنه)narrated:
"A liar is someone who lies in his oath. A miser is someone who is miserly with the greeting (of salam). A theif is someone who steals from the prayer (i.e. does not pray properly)."
One must always be gentle in their manner of greeting, as narrated in the hadith:
"Gentleness adorns every act. It's absence will tarnish it"
Jabir Ibn Abdullah (رضى الله عنه) narrated:
"Some of the Jews greeted the Prophet (صلى الله عليه وسلم) saying, As-sam 'alaykum (Death be upon you) and the Prophet responded, And upon you. 'Aisha (رضى الله عنها), said angrily "Didn't you hear what he said?" The Prophet replied Yes and I answered them. We will be answered for what we said of them and they will not be answered for what they said to us".
So even with the non-muslims one must remember the correct adab that must be adopted.
Giving salam in certain situations is disliked, for example when you may disturb someone, when being taught, listening to the khutbah of salat-ul-jumu'ah, eating, sleeping, or in an important or private situation for example. However in other circumstances we should take heed and give salam, the Prophet (صلى الله عليه وسلم) said:
"By the One in Whose hand in my soul, you will not enter Paradise until you believe, and you will not believe until you love one another. Shall I not tell you of something that if you do it, you will love one another? Spread salam amongst yourselves."
Imam Nawawi (رحمة الله عليه) said, "It is preferred to say Bismillah al-Rahman al-Rahim (In the name of Allah, Most Gracious, Most Merciful) when you enter your houses or the houses of others. You should say salam even if you enter vacant or uninhabited places and say a prayer to go out. It is narrated by Anas (رضى الله عنه) that the Prophet (صلى الله عليه وسلم) said:
"If someone says, 'In the name of Allah, I seek help from Allah. There is no strength nor might except with Allah', then he will be told ' You are protected my slave' and Satan will leave him."
(Tirmidhi & Abu Dawud)
He cited another hadith narrated by Muslim that Jabir Ibn Abdullah (رضى الله عنه) heard the Prophet (صلى الله عليه وسلم) say:
"If you enter your house and pray to Allah when entering and before your meals, Satan will say [to his minions] 'No sleep and no food'. If you enter without praying to Allah, Satan will say [to his minions], 'You have got your sleep and your meal'."NB: For further detail on the rulings related to "Greetings" see Chapter 2: "The Book of Manners"