Leviticus is the book of the Torah containing the principal commandments to observe Yom Kippur.
The most extensive passage is Leviticus 16, which describes the annual atonement ritual and the High Priest's service in the sanctuary. Leviticus 23:26–32 gives the festival legislation: Yom Kippur is the tenth day of the seventh month, a sacred occasion when people must “afflict” themselves and refrain from work.
In traditional Jewish interpretation, afflicting oneself includes fasting and several other restrictions. The Torah also specifies that the observance runs from the evening of the ninth day until the following evening, explaining why the holiday begins before sunset.
Numbers also mentions Yom Kippur in its list of festival offerings, so Leviticus is not the only Torah book that refers to the day. Nevertheless, Leviticus is the canonical answer because it contains the foundational laws and the detailed sanctuary ritual. Deuteronomy does not provide the primary Yom Kippur legislation.