Exodus 12:12-15

For I will go through the land of Egypt on that night, and will strike down all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgments—I am the LORD. The blood shall be a sign for you on the houses where you live; and when I see the blood I will pass over you, and no plague will befall you to destroy you when I strike the land of Egypt. ‘Now this day will be a memorial to you, and you shall celebrate it as a feast to the LORD; throughout your generations you are to celebrate it as a permanent ordinance. Seven days you shall eat unleavened bread, but on the first day you shall remove leaven from your houses; for whoever eats anything leavened from the first day until the seventh day, that person shall be cut off from Israel.