Exodus 12:7, 12, 13

Then they are to take some of the blood and put it on the sides and tops of the doorframes of the houses where they eat the lambs.

“On that same night I will pass through Egypt and strike down every firstborn of both people and animals, and I will bring judgment on all the gods of Egypt. I am the LORD.

The blood will be a sign for you on the houses where you are, and when I see the blood, I will pass over you. No destructive plague will touch you when I strike Egypt.