Sahih al-Bukhari 28
Sahih al-Bukhari · Book 2: Belief · To greet is a part of Islam
Narrated 'Abdullah bin 'Amr:
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 " .
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."