Exodus 12:12-13

For I will pass through the land of Egypt that night, and I will smite all the first-born 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, upon the houses where you are; and when I see the blood, I will pass over you, and no plague shall fall upon you to destroy you, when I smite the land of Egypt.