Parashas Metzora · Passuk 15:10
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 33a (verified on the daf).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: “That which is under him” — what is “under him”? If you say under the zav, that is already derived from “and a man who touches his bed”; rather: one who touches anything under which the zav is — and what is that? The upper surface of the zav (madaf).
Madaf impurity
Chazal identify a lighter impurity for items borne above the zav even without direct contact.