Moed Katan 8A

דהא עצים ואבנים בעלמא לא מטמאו, והכא מטמאו.

The Talmud explains that the halakha of leprous symptoms on a house constitutes a novelty, as by Torah law, wood and stones are generally not susceptible to ritual impurity, yet here in the case of house tzara'at they are susceptible to ritual impurity.

ורבי אמר: אצטריך, דאי כתב רחמנא ״וביום הראות בו״, הוה אמינא לדבר מצוה ⁠— אין, לדבר הרשות ⁠— לא, כתב רחמנא ״וצוה הכהן״.

And R' Yehuda HaNasi said: It was necessary for the Torah to state both verses: “But on the day it appears in him” (Leviticus 13:14), as well as: “Then the priest should command that they empty the house” (Leviticus 14:36). As, if God had written only: “But on the day it appears in him,” I would have said that for a matter of mitzva, yes, one may delay the priest’s examination of the leprous symptoms, but for an optional matter, no, one may not, in accordance with the opinion of R' Yehuda. Therefore God wrote: “Then the priest should command,” in order to teach that one may delay his examination even for an optional matter.

ואי כתב רחמנא ״וצוה הכהן״, הוה אמינא: הני ⁠— אין, דלאו טומאה דגופיה, אבל טומאה דגופיה ⁠— אימא מיחזא חזיא ליה. צריכא.

And conversely, if God had written only: “Then the priest should command,” I would have said that for these utensils that are found in the house, yes, one may delay the priest’s examination, as this is not a case of impurity of the individual’s body but only that of his possessions; but in a case of impurity of the individual’s body, I would say that the priest must examine it immediately. Therefore, it is necessary for the Torah to state that even in this case, one may delay his examination.