Parashas Metzora · Passuk 14:38
Chabad.org — English translation + Rashi
Torah Temima points to Chullin 10b (verified on the daf).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: “And the kohen shall go out …” — whence do the Sages derive the principle “leave the matter in its presumptive status”? Rav Shmuel bar Nachmani said in the name of R’ Yonatan: The verse says “and the kohen shall go out from the house to the entrance of the house and close it up.” Perhaps while he was going out and returning the size diminished? Rather, we must say that we leave it in its presumptive status.
Chazakah principle
Chazal derive the general rule of chazakah (presumptive status) from the fact that the kohen may close the house even though the nega might theoretically have changed size while he walked to the doorway.