Short answer
You can say good morning, but it does not replace the salam. The Islamic greeting is 'as-salamu alaykum', and starting with it is sunnah. The best way is to say salam first and then good morning. Replying to a salam is wajib, while replying to 'good morning' is good manners rather than an obligation.
Starting the greeting with salam is sunnah, and replying to it is wajib. Greetings such as 'good morning' or 'sabah al-khayr' are mubah, but using them instead of salam leaves the sunnah and its reward.
The four schools agree on this.
Explanation
The salam is not a cultural hello. It is a greeting Allah taught, a dua for peace, mercy and blessing, and the Quran calls it a greeting from Allah, blessed and good. The Prophet ﷺ told the Companions that spreading the salam is the way to love one another, and he counted greeting both people you know and people you do not as one of the best deeds in Islam.
'Good morning', 'good evening', 'hi' and the Arabic 'sabah al-khayr' (morning of goodness) are ordinary polite words. Nothing in them is haram. The issue is only what you lose if they replace the salam: the sunnah, the reward attached to it, and the bond it builds. So:
- With Muslims: say 'as-salamu alaykum' first, then 'good morning, how are you?' if you like.
- When someone says good morning to you: reply warmly. Allah says that when you are greeted, return it with something better or at least the same, so you could reply with the salam and a kind word.
- At work or school with non-Muslims: 'good morning' is a perfectly good greeting. See can you say salam to non-Muslims.
- In messages: write the salam in full rather than 'aslm' or 'AoA', so the dua is complete.
The reply to salam is wajib (obligatory): if one person in a group replies, the duty is met for all. The reply to 'good morning' is not wajib, but ignoring a greeting is poor manners. The sunnah order is that the one walking greets the one sitting, the smaller group greets the larger, and the younger greets the older, though whoever starts first is closest to Allah. The four schools agree on this. More in Assalamu alaikum and Walaikum assalam.
The evidence
Quran, Surah An-Nisaa 4:86
وَإِذَا حُيِّيتُم بِتَحِيَّةٍۢ فَحَيُّوا۟ بِأَحْسَنَ مِنْهَآ أَوْ رُدُّوهَآ ۗ إِنَّ ٱللَّهَ كَانَ عَلَىٰ كُلِّ شَىْءٍ حَسِيبًا ﴿٨٦﴾
And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it [in a like manner]. Indeed, Allah is ever, over all things, an Accountant.
Every greeting should be returned with one as good or better.
Quran, Surah An-Noor 24:61
لَّيْسَ عَلَى ٱلْأَعْمَىٰ حَرَجٌۭ وَلَا عَلَى ٱلْأَعْرَجِ حَرَجٌۭ وَلَا عَلَى ٱلْمَرِيضِ حَرَجٌۭ وَلَا عَلَىٰٓ أَنفُسِكُمْ أَن تَأْكُلُوا۟ مِنۢ بُيُوتِكُمْ أَوْ بُيُوتِ ءَابَآئِكُمْ أَوْ بُيُوتِ أُمَّهَٰتِكُمْ أَوْ بُيُوتِ إِخْوَٰنِكُمْ أَوْ بُيُوتِ أَخَوَٰتِكُمْ أَوْ بُيُوتِ أَعْمَٰمِكُمْ أَوْ بُيُوتِ عَمَّٰتِكُمْ أَوْ بُيُوتِ أَخْوَٰلِكُمْ أَوْ بُيُوتِ خَٰلَٰتِكُمْ أَوْ مَا مَلَكْتُم مَّفَاتِحَهُۥٓ أَوْ صَدِيقِكُمْ ۚ لَيْسَ عَلَيْكُمْ جُنَاحٌ أَن تَأْكُلُوا۟ جَمِيعًا أَوْ أَشْتَاتًۭا ۚ فَإِذَا دَخَلْتُم بُيُوتًۭا فَسَلِّمُوا۟ عَلَىٰٓ أَنفُسِكُمْ تَحِيَّةًۭ مِّنْ عِندِ ٱللَّهِ مُبَٰرَكَةًۭ طَيِّبَةًۭ ۚ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ لَكُمُ ٱلْءَايَٰتِ لَعَلَّكُمْ تَعْقِلُونَ ﴿٦١﴾
There is not upon the blind [any] constraint nor upon the lame constraint nor upon the ill constraint nor upon yourselves when you eat from your [own] houses or the houses of your fathers or the houses of your mothers or the houses of your brothers or the houses of your sisters or the houses of your father's brothers or the houses of your father's sisters or the houses of your mother's brothers or the houses of your mother's sisters or [from houses] whose keys you possess or [from the house] of your friend. There is no blame upon you whether you eat together or separately. But when you enter houses, give greetings of peace upon each other - a greeting from Allah, blessed and good. Thus does Allah make clear to you the verses [of ordinance] that you may understand.
The salam is the greeting Allah taught the believers.
Sahih Muslim 54
Sahih... أولا أدلكم على شيء إذا فعلتموه تحاببتم أفشوا السلام بينكم ...
"Shall I not tell you of something which, if you do it, you will love one another? Spread the salam among you."
Spreading the salam builds love between Muslims.
Sahih al-Bukhari 12
Sahih... تطعم الطعام وتقرأ السلام على من عرفت ومن لم تعرف ...
"Feed people, and give the salam to those you know and those you do not know."
Greeting with salam is among the best deeds in Islam.
Sunan Abi Dawud 5197
Grading to be checked... إن أولى الناس بالله من بدأهم بالسلام ...
"The closest of people to Allah is the one who starts the salam."
Starting the greeting carries special virtue.
Where the ruling is recorded
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on salam and replying to a sneeze. Starting the salam is sunnah and replying is wajib, a duty met for the group by one reply.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.