Exodus 12:12-14, 23

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. 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. And this day shall be unto you for a memorial, and ye shall keep it a feast to Jehovah: throughout your generations ye shall keep it a feast by an ordinance for ever.

For Jehovah will pass through to smite the Egyptians; and when he seeth the blood upon the lintel, and on the two side-posts, Jehovah will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you.