Exodus 12:7, 13

7.) And they shall take of the blood, and strike it on the two side posts and on the upper door post of the houses, wherein they shall eat it.

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

Click here to see this verse in context