Short answer
Yes. Starting the salam is sunnah, but replying is wajib. When someone says Assalamu alaikum, reply at least Wa alaikum assalam, and better is to add wa rahmatullahi wa barakatuh. In a group, one person's reply is enough for everyone, and the reply should be loud enough to be heard.
Initiating the greeting is sunnah and replying is wajib, as a collective duty when a group is greeted. The reply must be audible to the greeter. Replying to a written salam is also wajib, in writing or by saying it.
The four schools agree on this.
Explanation
The Quran says: when you are greeted, reply with something better or at least return it. The Prophet ﷺ taught that the rider greets the walker, the walker greets the one sitting, and the smaller group greets the larger one. A man greeted him with Assalamu alaikum and he said 'ten'; another added wa rahmatullah and he said 'twenty'; a third added wa barakatuh and he said 'thirty', meaning rewards.
The reply:
- Greeting: Assalamu alaikum wa rahmatullahi wa barakatuh.
- Reply: Wa alaikum assalam wa rahmatullahi wa barakatuh.
- The minimum is Wa alaikum assalam. Replying with only 'hi' or a nod does not fulfil the duty.
When you do not have to reply:
- while praying, reciting the Quran in a gathering, giving or listening to the khutbah, or calling the adhan;
- while using the toilet;
- while eating with food in your mouth.
Other common cases. A salam passed on by someone ('My mother sends you salam') is answered with Wa alaikum wa alaiha assalam. A salam in a message or email is answered in writing or aloud. Between a non-mahram man and young woman, the Hanafi scholars advise greeting briefly or replying quietly where there is risk of fitnah. Salam to non-Muslims has its own answer: see salam to non-Muslims. Spellings and meanings are on Islamic phrases, and if you are wondering about 'good morning', see good morning instead of salam. The four schools agree that replying is obligatory.
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.
Returning a greeting is commanded.
Sahih al-Bukhari 6232
Sahih... يسلم الراكب على الماشي والماشي على القاعد والقليل على الكثير ...
"The rider greets the walker, the walker greets the one sitting, and the few greet the many."
Shows who should start the greeting.
Jami' at-Tirmidhi 2689
Grading to be checked... ثم جاء آخر فقال السلام عليكم ورحمة الله ...
"Then another came and said, 'Peace be upon you and the mercy of Allah', and he said, 'Twenty.'"
Fuller greetings and replies bring more reward.
Where the ruling is recorded
- al-Durr al-Mukhtar, Kitab al-Hazr wa al-Ibaha, section on greetings. Starting salam is sunnah, replying is wajib, and situations where no reply is due.
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on greetings. Replying to a written salam and to a salam passed on by someone.
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.