God: Preserver in Saving the Firstborn, when the Plague of Death Destroyed the Firstborn of Egypt,

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

When the LORD passes through to strike down the Egyptians, He will see the blood on the top and sides of the doorframe and will pass over that doorway; so He will not allow the destroyer to enter your houses and strike you down.