Rashi on Leviticus 15:6/רש"י על ויקרא ט״ו:ו׳

1והישב על הכלי AND HE THAT SITTETH ON ANY THING [WHEREON HE SAT THAT HATH AN ISSUE … SHALL BE UNCLEAN] — even though he (the person) is not actually in contact with it: even if there be ten things one upon the other, the lowest being that upon which a זב has once sat, they all communicate uncleanness according to the law applicable to a seat upon which an unclean person has sat, and similarly is the case with an object upon which he has lain (cf. Sifra, Metzora Parashat Zavim, Chapter 3 1).