Exodus 12:13 in the King James Version
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.
Context
12 For I will go through the land of Egypt in that night, and will strike all the firstborn in the land of Egypt, both man and animal. I will execute judgments against all the gods of Egypt. I am the LORD.
13 The blood shall be to you for a token on the houses where you are. When I see the blood, I will pass over you, and no plague will be on you to destroy you when I strike the land of Egypt.
14 This day shall be a memorial for you. You shall keep it as a feast to the LORD. You shall keep it as a feast throughout your generations by an ordinance forever.