Exodus 12:12-13

On that night I will pass through the land of Egypt and strike down every firstborn son and firstborn male animal in the land of Egypt. I will execute judgment against all the gods of Egypt, for I am the LORD! But the blood on your doorposts will serve as a sign, marking the houses where you are staying. When I see the blood, I will pass over you. This plague of death will not touch you when I strike the land of Egypt.