מדרש על ויקרא 16:30

1) (Vayikra 16:30) ("For on this day He shall atone for you to cleanse you of all of your sins; before the L–rd you shall be clean.") "For on this day He shall atone for you": by the offerings. And whence is it derived that even without offerings and without the he-goat, the day (itself) atones? From "For on this day He shall atone." For transgressions between man and G d Yom Kippur atones; for transgressions between man and his neighbor Yom Kippur does not atone until he conciliates his neighbor.

2) Thus did R. Elazar b. Azaryah expound: "Of all of your sins before the L–rd you shall be clean": For things between yourself and G d you are pardoned. For things between yourself and your neighbor you are not pardoned until you conciliate your neighbor.

3) And whence is it derived that eating, drinking, bathing, anointment, shodding, and cohabitation are forbidden on Yom Kippur? From "shabbaton shvuth." I might then think that all of these are forbidden on the Sabbath of creation (i.e., an ordinary Sabbath). It is, therefore, written (Vayikra 16:31) "Sabbath shabbaton is it" (Yom Kippur). It is forbidden in all of them, but not the Sabbath of creation.