Sahih Bukhari Volume 001, Book 002, Hadith Number 027.

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."

source : www.hadithcollection.com