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."

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 ".

Grade: Sahih (authentic)
Reference : Sahih al-Bukhari 28
In-book reference : Book 2, Hadith 21
USC-MSA web (English) reference : Vol. 1, Book 2, Hadith 28
(deprecated numbering scheme)

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Abdullah ibn Amr reported: A man asked the Messenger of Allah, peace and blessings be upon him, “Which Islam is best?” The Prophet said, “To feed the hungry and to greet with peace those you know and those you do not know.”

Source: Sahih Bukhari 28

Grade: Sahih (authentic) according to Al-Bukhari

For ARABIC, click here:

http://dailyhadith.abuaminaelias.com/2012/09/10/hadith-on-islam-the-best-muslims-greet-with-peace-those-whom-they-know-and-do-not-know/