Belief
2/20. Chapter:
To greet is a part of Islam
وَقَالَ عَمَّارٌ: ثَلاَثٌ مَنْ جَمَعَهُنَّ فَقَدْ جَمَعَ الإِيمَانَ الإِنْصَافُ مِنْ نَفْسِكَ، وَبَذْلُ السَّلاَمِ لِلْعَالَمِ، وَالإِنْفَاقُ مِنَ الإِقْتَارِ.And 'Ammar said, "Whoever acquires the following three qualities will acquire faith: 1. To treat others as one likes to be treated by others. 2. To greet everybody (known and unknown). 3. To spend (give charitable gifts) in Allah's Cause, in spite of poverty."
Sahih Bukhari
حَدَّثَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ يَزِيدَ بْنِ أَبِي حَبِيبٍ، عَنْ أَبِي الْخَيْرِ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، أَنَّ رَجُلاً، سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم أَىُّ الإِسْلاَمِ خَيْرٌ قَالَ " تُطْعِمُ الطَّعَامَ، وَتَقْرَأُ السَّلاَمَ عَلَى مَنْ عَرَفْتَ وَمَنْ لَمْ تَعْرِفْ ".
Narrated 'Abdullah bin 'Amr: A person asked Allah's Messenger (ﷺ) . "What (sort of) deeds in or (what qualities of) Islam are good?" He replied, "To feed (the poor) and greet those whom you know and those whom you don't know."
Reference : Sahih al-Bukhari 28In-book reference : Book 2, Hadith 21USC-MSA web (English) reference : Vol. 1, Book 2, Hadith 28