✅Answer: Pay attention to the following narration:

عَنِ الْحَسَنِ بْنِ الْمُنْذِرِ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ يَقُولُ مَنْ قَالَ السَّلَامُ عَلَيْكُمْ فَهِيَ عَشْرُ حَسَنَاتٍ وَ مَنْ قَالَ سَلَامٌ عَلَيْكُمْ وَ رَحْمَةُ اللَّهِ فَهِيَ عِشْرُونَ حَسَنَةً وَ مَنْ قَالَ سَلَامٌ عَلَيْكُمْ وَ رَحْمَةُ اللَّهِ وَ بَرَكَاتُهُ فَهِيَ ثَلَاثُونَ حَسَنَةً.

📚 Al-Kafi, vol. 2, p. 645

Translation: Hasan ibn Mundhir said: I heard Imam Sadiq, peace be upon him, saying: Whoever says 'Salamun 'alaykum' (peace be upon you) when greeting, gets ten good deeds, and whoever says 'Salamun 'alaykum wa rahmatullah' (peace be upon you and the mercy of Allah), gets twenty good deeds, and whoever says 'Salamun 'alaykum wa rahmatullahi wa barakatuh' (peace be upon you and the mercy of Allah and His blessings), gets thirty good deeds (for him).

✅ There are numerous narrations in the Shia hadith collections regarding the manner of greeting and responding to a greeting, some of which are meant to explain the manner of greeting, and most of them are the practice and method of the Ahl al-Bayt, peace be upon them, in greeting and responding to a greeting, but in all of them, 'salam' is accompanied by the preposition 'alaika' or 'alaikum', and 'salam' alone was not in the tradition and method and instructions of the Ahl al-Bayt, peace be upon them, but unfortunately, today many people suffice with only the word 'salam' for both greeting and responding, and this is not in accordance with the teachings of the Ahl al-Bayt, peace be upon them, and it is necessary for people to return to the tradition and benefit from the great reward of correctly greeting a believer and a Muslim.

✅ Also, pay attention to the following hadith:

عَنْ زُرَارَةَ عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: تَقُولُ فِي الرَّدِّ عَلَى الْيَهُودِيِّ وَ النَّصْرَانِيِّ سَلَامٌ.

📚 Al-Kafi, vol. 2, p. 649

Translation: Imam Sadiq, peace be upon him, said: In response to the greeting of a Jew or a Christian, you say: 'Salam'.

✅ We must be careful not to treat the believers and Muslims in a way that is only appropriate for a Jew or a Christian, because to suffice with only 'salam' without saying 'alaika' or 'alaikum' is specific to the response to the greeting of a Jew or a Christian.

✅ This hadith is also reported for responding to a Jew or a Christian:

عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: قَالَ أَمِيرُ الْمُؤْمِنِينَ لَا تَبْدَءُوا أَهْلَ الْكِتَابِ بِالتَّسْلِيمِ وَ إِذَا سَلَّمُوا عَلَيْكُمْ فَقُولُوا وَ عَلَيْكُمْ.

📚 Al-Kafi, vol. 2, p. 648

Translation: Imam Sadiq, peace be upon him, said: Amir al-Mu'minin, peace be upon him, said: Do not initiate the greeting to the People of the Book (i.e., Jews and Christians), and if they greet you, respond to them by saying: 'wa 'alaykum' (and upon you).

✅ Therefore, responding to a greeting with only 'salam' or with only 'alaikum' is for a Jew or a Christian and should not be used for believers and Muslims.

✅ Greeting and responding to a greeting have different manners and rulings that can be provided upon request. If Allah wills.

✅ Also refer to question 218 of the channel. If Allah wills.