Exodus 12:12-15

For I [the LORD] will pass through the land of Egypt on this night, and will strike down all the firstborn in the land of Egypt, both man and animal; against all the gods of Egypt I will execute judgments [exhibiting their worthlessness]. I am the LORD. The blood shall be a sign for you on [the doorposts of] the houses where you live; when I see the blood I shall pass over you, and no affliction shall happen to you to destroy you when I strike the land of Egypt. [1 Cor 5:7; Heb 11:28] ‘Now this day will be a memorial to you, and you shall keep it as a feast to the LORD; throughout your generations you are to celebrate it as an ordinance forever. [In the celebration of the Passover in future years,] seven days you shall eat unleavened bread, but on the first day you shall remove the leaven from your houses [because it represents the spread of sin]; for whoever eats leavened bread on the first day through the seventh day, that person shall be cut off and excluded from [the atonement made for] Israel.