From Sahih Bukhari (Hadith 12):
Narrated 'Abdullah bin 'Amr: A man asked the Prophet (ﷺ) , "What sort of deeds or (what qualities of) Islam are good?" The Prophet (ﷺ) replied, 'To feed (the poor) and greet those whom you know and those whom you do not Know (See Hadith No)
Reference: Bukhari 12