Mishnah Negaim 9:2: [The Negaim within] a boil and a burn do not combine with each other [to make a Nega large enough to cause impurity], and do not spread from one to another, and do not spread to [normal] skin, and [normal] skin does not spread into them. If they were unhealed, they are pure [even with a Nega]. If they formed a membrane like the skin of garlic, this is the scab of the boil mentioned in the Torah. [If] they returned [to normal] and healed, even though their area is scarred, they are judged like [normal] skin.

הַשְּׁחִין וְהַמִּכְוָה אֵין מִצְטָרְפִין זֶה עִם זֶה, וְאֵין פּוֹשִׂין מִזֶּה לָזֶה, וְאֵין פּוֹשִׂין לְעוֹר הַבָּשָׂר, וְלֹא עוֹר הַבָּשָׂר פּוֹשֶׂה לְתוֹכָן. הָיוּ מוֹרְדִין, טְהוֹרִין. עָשׂוּ קְרוּם כִּקְלִפַּת הַשּׁוּם, זוֹ הִיא צָרֶבֶת הַשְּׁחִין הָאֲמוּרָה בַתּוֹרָה. חָזְרוּ וְחָיוּ, אַף עַל פִּי שֶׁמְּקוֹמָן צַלֶּקֶת, נִדּוֹנִין כְּעוֹר הַבָּשָׂר:

Mishnah Negaim 9:3: They asked Rabbi Eliezer: One whose palm raised a Baheret like [the size of] a Sela [coin] and the area of it [the palm] is a scabbed boil, [what is the law]? He responded to them: He should be quarantined. They responded to him: Why? [The palm] is not fit for growing white hair, and as for spreading - it does not spread, and through a healthy patch of skin it does not become impure! He said to them: Maybe it will recede and then spread. They said to him: But is not its area like [the size of] a bean! He said to them: I did not learn [this case]. Rabbi Yehuda ben Beteira said to him: I will teach it. He said to him: If you will confirm the words of the Sages [that he is confined], go on. He said to him: [The reason is] maybe he will develop another boil outside of it and it will [then] spread into it. He said to him: You are a great wise man that you have confirmed the words of the Sages!

שָׁאֲלוּ אֶת רַבִּי אֱלִיעֶזֶר, מִי שֶׁעָלְתָה לְתוֹךְ יָדוֹ בַּהֶרֶת כַּסֶּלַע וּמְקוֹמָהּ צָרֶבֶת שְׁחִין. אָמַר לָהֶם, יַסְגִּיר. אָמְרוּ לוֹ, לָמָּה, לְגַדֵּל שֵׂעָר לָבָן אֵינָהּ רְאוּיָה, וּלְפִשְׂיוֹן אֵינָהּ פּוֹשָׂה, וּלְמִחְיָה אֵינָהּ מִטַּמָּא. אָמַר לָהֶן, שֶׁמָּא תִכְנֹס וְתִפְשֶׂה. אָמְרוּ לוֹ, וַהֲלֹא מְקוֹמָהּ כַּגְּרִיס. אָמַר לָהֶן, לֹא שָׁמַעְתִּי. אָמַר לוֹ רַבִּי יְהוּדָה בֶן בְּתֵירָא, אֲלַמֵּד בּוֹ. אָמַר לוֹ, אִם לְקַיֵּם דִּבְרֵי חֲכָמִים, הֵין. אָמַר לוֹ, שֶׁמָּא יִוָּלֵד לוֹ שְׁחִין אַחֵר חוּצָה לוֹ וְיִפְשֶׂה לְתוֹכוֹ. אָמַר לוֹ, חָכָם גָּדוֹל אַתָּה שֶׁקִּיַּמְתָּ דִּבְרֵי חֲכָמִים: