exodus 12:22 And ye shall take a bunch of hyssop, and dip it in the blood that is in the bason, and strike the lintel and the two side posts with the blood that is in the bason; and none of you shall go out at the door of his house until the morning. - Free Bible Online

      22. hyssop--a small red moss [HASSELQUIST]; the caper-plant [ROYLE]. It was used in the sprinkling, being well adapted for such purposes, as it grows in bushes--putting out plenty of suckers from a single root. And it is remarkable that it was ordained in the arrangements of an all-wise Providence that the Roman soldiers should undesignedly, on their part, make use of this symbolical plant to Christ when, as our Passover, He was sacrificed for us [Joh 19:29].
      none . . . shall go out at the door of his house until the morning--This regulation was peculiar to the first celebration, and intended, as some think, to prevent any suspicion attaching to them of being agents in the impending destruction of the Egyptians; there is an allusion to it (Isa 26:20).

JFB.