Sh'mot (Exo) 12:12-13

For that night, I will pass through the land of Egypt and kill all the firstborn in the land of Egypt, both men and animals; and I will execute judgment against all the gods of Egypt; I am ADONAI. The blood will serve you as a sign marking the houses where you are; when I see the blood, I will pass over you — when I strike the land of Egypt, the death blow will not strike you.