Exodus 12:21-22

21Then Moses called for all the elders of Israel and said to them, “Go and take a lamb for yourselves according to [the size of] your families and slaughter the Passover lamb.

22You 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.