Exodus 12:21-24

Then Moses called all the elders of Israel together and told them, “Get the animals for your families and kill the lamb for the Passover. Take a branch of the hyssop plant, dip it into the bowl filled with blood, and then wipe the blood on the sides and tops of the doorframes. No one may leave that house until morning. When the LORD goes through Egypt to kill the Egyptians, he will see the blood on the sides and tops of the doorframes, and he will pass over that house. He will not let the one who brings death come into your houses and kill you. “You must keep this command as a law for you and your descendants from now on.