Exodus 12:7, 12-13

The people must take some of the blood and put it on the sides and tops of the doorframes of the houses where they eat the lambs.

“That night I will go through the land of Egypt and kill all the firstborn animals and people in the land of Egypt. I will also punish all the gods of Egypt. I am the LORD. But the blood will be a sign on the houses where you are. When I see the blood, I will pass over you. Nothing terrible will hurt you when I punish the land of Egypt.