Exodus 12:12-13

12“I will pass through the land of Egypt on that night and strike every firstborn male in the land of Egypt, both people and animals. I am the Lord; I will execute judgments against all the gods of Egypt.

13The blood on the houses where you are staying will be a distinguishing mark for you; when I see the blood, I will pass over you. No plague will be among you to destroy you when I strike the land of Egypt.