Midrasz do Kapłańska 15:4

1) (Vayikra 15:4) ("Every [object of] lying which the zav lies down upon shall be tamei, and every implement of sitting which he sits upon shall be tamei.") "which the zav lies down upon": I might think even if he lies on a pit cover or on a door; it is, therefore, written (object of) "lying"; not on a pit cover and not on a door.

2) I would exclude these, but I would not exclude a mat of reeds or shoots; it is, therefore written "the lying" (connoting something which is) customarily used (for lying on).

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.