Exodus 12:21-23

Then 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. 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.