Exodus 12:22-24

Take a bunch of hyssop, dip it into the blood in the basin and put some of the blood on the top and on both sides of the door-frame. None of you shall go out of the door of your house until morning. When the LORD goes through the land to strike down the Egyptians, he will see the blood on the top and sides of the door-frame and will pass over that doorway, and he will not permit the destroyer to enter your houses and strike you down. ‘Obey these instructions as a lasting ordinance for you and your descendants.