The Book of Exodus describes the first Passover in chapter 12. Its opening chapters recount the Israelites’ enslavement in Egypt and Moses’ mission to confront Pharaoh. Exodus 12 then gives the instructions for the first Passover: select an unblemished lamb, slaughter it, place its blood on the doorposts, and eat it with unleavened bread and bitter herbs.
The chapter connects the ritual to the tenth plague, when the firstborn of Egypt die but the marked Israelite homes are spared. It also commands that the event be remembered as an annual observance. This is why Exodus is both the name of the second book of the Torah and the name commonly given to the Israelites’ departure from Egypt.
Genesis comes immediately before Exodus and explains the family history that brings Jacob’s descendants to Egypt. Numbers and Leviticus contain later laws and references to Passover, but neither records the first Passover narrative in chapter 12.