Parashas Metzora · Passuk 14:46
Chabad.org — English translation + Rashi
Torah Temima points to Shevuos 17b (verified on the daf).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: “And whoever enters the house” — R’ Oshaya said: One who enters a house afflicted with a nega by walking backwards, even if his entire body is inside except for his nose, remains pure, for it is written “and whoever enters the house” — the Torah forbade [only] the normal manner of entry.
Manner of entry
Chazal limit the impurity of entering an afflicted house to ordinary forward entry; unusual entry (backwards) does not convey the same tumah.