Exodus 12:12-13

12I will pass through the land of Egypt in the same night, and I will attack all the firstborn in the land of Egypt, both of humans and of animals, and on all the gods of Egypt I will execute judgment. I am the Lord.

13The blood will be a sign for you on the houses where you are, so that when I see the blood I will pass over you, and this plague will not fall on you to destroy you when I attack the land of Egypt.