Midrash su Levitico 15:5

1) (Vayikra 15:5) ("And a man who touches what he lay upon shall wash his clothes and bathe in water, and he shall be unclean until the evening.") A man who touches a zav's couch confers tumah to his garments; but a zav's couch that touches (another) couch does not confer tumah to garments (lying upon it). (For if not for the verse,) would it not follow a fortiori that it (the other couch) should confer tumah to garments, viz.: If in a place where a man lying under a zav does not become tamei to confer tumah upon men and garments, a couch under a zav does become tamei to confer tumah upon men and garments, then in a place where a man becomes tamei by touching his couch to confer tumah to his garments, how much more so should a couch become tamei by touching a zav's couch to confer tumah to garments (lying upon it)! It must, therefore, be written: "And a man who touches what he lay upon shall wash his clothes" — A man who touches a zav's couch confers tumah to his garments; but a zav's couch that touches (another) couch does not confer tumah to garments (lying upon it).

2) (But now we have) an a fortiori argument that a man lying under a zav should become tamei to confer tumah to men and garments! viz.: If in a place where a couch does not become tamei by touching a zav's couch to confer tumah to garments (lying upon it), a man does become tamei by touching a zav's couch to confer tumah to his garments, then in a place where a couch becomes tamei under a zav to confer tumah to a man and his garments, how much more so should a man lying under a zav become tamei to confer tumah to men and garments! It is, therefore, written (Vayikra 15:4) "Every couch on which a zav lies … shall be unclean." A couch becomes tamei under a zav to confer tumah to men and garments, but a man lying under a zav does not become tamei to confer tumah to men and garments.

3) "And a man who touches his (a zav's) couch": (It should have been written) "the couch" (and not "his couch," and we would understand from the context that it is his couch. It must come to teach us then) that if it breaks (after the zav has lain on it [so that it is no longer "his couch," that he has lain on]), it is tahor. (Why is a verse needed for this?) Does it not follow a fortiori, viz.: If an earthen vessel, which cannot be cleaned of its tumah (in a mikvah), becomes tahor if it breaks, then a zav's couch, which can be cleansed of its tumah, how much more so should it be tahor if it breaks! — No, this may be true of an earthen vessel, which does not become an av (i.e., a progenitor) of tumah to confer tumah upon men and vessels, as opposed to the zav's couch, which does become an av of tumah to confer tumah upon men and vessels. Since this is the case, we would say that if it breaks, it does not become tahor. It must, therefore, be written "his couch," (to indicate that) if it broke (after he lay upon it), it is tahor.