Exodus 12:22-24

22And ye shall take a bunch of hyssop, and dip it in the blood that is in the basin, and strike the lintel and the two side posts with the blood that is in the basin; and none of you shall go out at the door of his house till the morning. 23For the LORD will pass through to smite the Egyptians; and when he seeth the blood upon the lintel, and on the two side posts, the LORD will pass over the door, and will not permit the destroyer to come in to your houses to smite you . 24And ye shall observe this thing for an ordinance to thee and to thy sons for ever.