Midrash su Levitico 15:23

14) (Vayikra 15:23) ("And if on the bed [mishkav] he be, or on the object that she sits upon, when he touches it, he shall be unclean until the evening.") "And if on the mishkav," even only part of it "he be (even without directly touching it)," so that most of him is borne by it, "or on the object," even only part of it," that she sits upon." From here they ruled: If most of the tamei were over the (object that is) tahor, or over part of it; or if most of the tahor were over the tamei or over part of it — whether zav or mishkav — (the ruling is) tamei. If part of the tamei were over the tahor or over part of it, or part of the tahor over the tamei or over part of it — with a zav (vis-à-vis a man), he is tamei; with (a zav vis-à-vis) a mishkav, (the mishkav is) tahor. R. Shimon says: Even with part tamei over tahor, even with zav, (the ruling is) tahor.

15) (Vayikra 15 verse 23) "mishkav": This is mishkav (itself); "which she sits upon": This is moshav; "object": This is merkav ( a saddle), it being written "when he touches it." Respecting which object was a distinction made between touching it and carrying it, (the former rendering one tamei; the latter, not)? A saddle.