Exodus 12:21-23

Then Moses called for all the elders of Israel and said to them, “Pick out and take lambs for yourselves according to your families, and kill the Passover lamb. And you shall take a bunch of hyssop, dip it in the blood that is in the basin, and strike the lintel and the two doorposts with the blood that is in the basin. And none of you shall go out of 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 and on the two doorposts, the LORD will pass over the door and not allow the destroyer to come into your houses to strike you.