Sahih Al Bukhari 1: Chapter 2, Hadith 28

(28) To greet is a part of Islam

حَدَّثَنَا قُتَيْبَةُ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا اللَّيْثُ، ‏‏‏‏‏‏عَنْ يَزِيدَ بْنِ أَبِي حَبِيبٍ، ‏‏‏‏‏‏عَنْ أَبِي الْخَيْرِ، ‏‏‏‏‏‏عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، ‏‏‏‏‏‏أَنَّ رَجُلًا سَأَل رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏أَيُّ الْإِسْلَامِ خَيْرٌ؟ قَالَ:‏‏‏‏ "تُطْعِمُ الطَّعَامَ، ‏‏‏‏‏‏وَتَقْرَأُ السَّلَامَ عَلَى مَنْ عَرَفْتَ وَمَنْ لَمْ تَعْرِفْ".

SAHIH HADITH:

Narrated 'Abdullah bin 'Amr: A person asked Allah's Apostle . 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.