Exodus 12:12-13
12 For I will go through the land of Egypt in that night, and will smite all the first-born in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgments: I am Jehovah.
13 And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and there shall no plague be upon you to destroy you, when I smite the land of Egypt.