Exodus 12:13

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.

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 the plague shall not be upon you to destroy you, when I smite the land of Egypt.