Sahih Bukhari Volume 001, Book 002, Hadith Number 027.
Sahih Bukhari Book 02. Belief
Narated By '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."