Exodus 12:21-23

Moses assembled all the elders of Israel. He said, “Select a lamb for your families and slaughter the Passover lamb. Take a bunch of hyssop and dip it in the bowl of blood and smear it on the lintel and on the two doorposts. No one is to leave the house until morning. GOD will pass through to strike Egypt down. When he sees the blood on the lintel and the two doorposts, GOD will pass over the doorway; he won’t let the destroyer enter your house to strike you down with ruin.