Exodus 12:21-22

21Then 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.

22Drain 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.