Adam Clarke on Exodus 12:7

Take of the blood, and strike it on the two side posts - This was to be done by dipping a bunch of hyssop into the blood, and thus sprinkling it upon the posts, etc.; see Exo 12:22. That this sprinkling of the blood of the paschal lamb was an emblem of the sacrifice and atonement made by the death of Jesus Christ, is most clearly intimated in the sacred writings, 1Pet 1:2; Heb 9:13, Heb 9:14; Heb 8:10. It is remarkable that no blood was to be sprinkled on the threshold, to teach, as Mr. Ainsworth properly observes, a reverent regard for the blood of Christ, that men should not tread under foot the Son of God, nor count the blood of the covenant wherewith they were sanctified an unholy thing; Heb 10:29.