Exodus 12:13; Exodus 12:23

13 The blood shall be a sign for you, on the houses where you are. And when I see the blood, I will pass over you, and no plague will befall you to destroy you, when I strike the land of Egypt.

23 For the LORD will pass through to strike the Egyptians, and when he sees the blood on the lintel and on the two doorposts, the LORD will pass over the door and will not allow the destroyer to enter your houses to strike you.