The Ka'ba is the place of worship which God commanded Abraham and Ishmael to build over four thousand years ago. The building was constructed of stone on what many believe was the original site of a sanctuary established by Adam. God commanded Abraham to summon all mankind to visit this place, and when pilgrims go there today they say 'At Thy service, O Lord', in response to Abraham's summons.
"When a person dies, his works end, except for three: ongoing charity, knowledge that is benefited from, and a righteous child who prays for him."
Prophet Mohammed (PBUH)
"The best of what a man leaves behind are three: a righteous child who supplicates for him, ongoing charity the reward of which reaches him, and knowledge that is acted upon after him."
Sunan Ibn Mājah
"Every day two angels come down from Heaven and one of them says, 'O Allah! Compensate every person who spends in Your Cause,' and the other (angel) says, 'O Allah! Destroy every miser.'"
Sahih Bukhari