Exodus 12:21-23

Then Moses called all the elders of Israel together and said to them, “Go, pick out a lamb or young goat for each of your families, and slaughter the Passover animal. Drain the blood into a basin. Then take a bundle of hyssop branches and dip it into the blood. Brush the hyssop across the top and sides of the doorframes of your houses. And no one may go out through the door until morning. For the LORD will pass through the land to strike down the Egyptians. But when he sees the blood on the top and sides of the doorframe, the LORD will pass over your home. He will not permit his death angel to enter your house and strike you down.