Exodus 12:22-24

22And take a bunch of hyssop, and dip [it] in the blood that is in the bason, and smear the lintel and the two door-posts with the blood that is in the bason; and none of you shall go out of the door of his house until the morning.

23And Jehovah will pass through to smite the Egyptians; and when he sees the blood on the lintel, and on the two door-posts, Jehovah will pass over the door, and will not suffer the destroyer to come into your houses to smite [you].

24And ye shall observe this as an ordinance for thee and for thy sons for ever.