מדרש אויף ויקרא 15:21

11) (Vayikra 15:20) "And all that she lies upon in her niddah state shall be unclean, and all that she sits upon shall be unclean": I might think that she confers tumah upon a mishkav that is not distinctive for reclining upon and upon a moshav (a seat) that is not distinctive for sitting upon. It is, therefore, written (Vayikra 15:21) "And whoever touches her (distinctive) mishkav shall wash his clothes."

12) (Vayikra 15:22) "And whoever touches any object that she sits upon shall wash his clothes": Just as below (i.e., Vayikra 15 verse 21, which elucidates verse 20), she confers tumah only upon a mishkav that is distinctive for lying upon and a moshav that is distinctive for sitting upon, here, too, she confers tumah only upon a mishkav that is distinctive for lying upon and a moshav that is distinctive for sitting upon.