Exodus 12:24

22You shall take a bunch of hyssop, and dip it in the blood that is in the basin, and strike the lintel and the two door posts with the blood that is in the basin. None of you shall go out of the door of his house until the morning.

23For the LORD will pass through to strike the Egyptians; and when he sees the blood on the lintel, and on the two door posts, the LORD will pass over the door, and will not allow the destroyer to come in to your houses to strike you.

24You shall observe this thing for an ordinance to you and to your sons forever.

25It shall happen when you have come to the land which the LORD will give you, as he has promised, that you shall keep this service.

26It will happen, when your children ask you, ‘What do you mean by this service?’