Midrash sur Le Lévitique 15:26

10) (Vayikra 15:26) ("Every mishkav which she lies upon all the days of her flow, as the mishkav of her niddah period shall be to her. And every object that she sits upon shall be unclean, as the uncleanliness of her niddah period.") As the (degree of) tumah of her niddah period, and not as the (number of) days of her niddah period. For (without the verse) would it not follow a fortiori (that the object is tamei the same number of days as the niddah!), viz.: Now if in a place where the tumah of the "conferred upon" (i.e., the man) was not made equivalent to (the severity of) the tumah of the conferrer (i.e., the niddah), the (number of) days of the conferred upon were made equivalent to the days (seven) of the conferrer, then here, where (the degree of severity of) the tumah of the conferred upon was made equivalent to that of the conferrer, how much more so should the days of the conferred upon be made equivalent to those of the conferrer! It is, therefore, written: "as the tumah of her niddah period." As the tumah of her niddah period, and not as the days of her niddah period. (Vayikra 15:27) "And whoever touches them shall be unclean": even their projections (those of the zav or of the aforementioned objects).