Sahih Muslim 39
Sahih Muslim · Book 1: The Book of Faith · Clarifying the superiority of Islam, and what part of it is best
It is narrated on the authority of 'Abdullah b. 'Amr that a man asked the Messenger of Allah (may peace and blessings be upon him) which of the merits (is superior) in Islam. He (the Holy Prophet) remarked:
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا لَيْثٌ ، ح وَحَدَّثَنَا مُحَمَّدُ بْنُ رُمْحِ بْنِ الْمُهَاجِرِ ، أَخْبَرَنَا 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 " .
That you provide food and extend greetings to one whom you know or do not know.