Exodus 12:12-14, 23

For I will pass through the land of Egypt that night, and I will strike all the firstborn in the land of Egypt, both man and beast; and on 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 are. 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. “This day shall be for you a memorial day, and you shall keep it as a feast to the LORD; throughout your generations, as a statute forever, you shall keep it as a feast.

For the LORD will pass through to strike the Egyptians, and when he sees the blood on the lintel and on the two doorposts, the LORD will pass over the door and will not allow the destroyer to enter your houses to strike you.