Exodus 12:22-24

You shall take a bunch of hyssop, dip it in the blood which is in the basin, and touch some of the blood to the lintel [above the doorway] and to the two doorposts; and none of you shall go outside the door of his house until morning. For the LORD will pass through to strike the Egyptians; and when He sees the blood on the lintel [above the entry way] and on the two doorposts, the LORD will pass over the door and will not allow the destroyer to come into your houses to slay you. You shall observe this event [concerning Passover] as an ordinance for you and for your children forever.