Leviticus 15:6

And he that sitteth on [any] thing whereon he sat that hath the
issue
Shall be unclean, even though he does not touch it. Jarchi says, though there should be, as he adds, ten things or vessels one upon another, they all defile because of sitting, and so by lying:

shall wash his clothes, and bathe [himself] in water, and be unclean
until the even; as in the preceding case. (See Gill on Leviticus 15:5).