Mishnah Negaim 4:11: A Baheret like [the size of] half a bean with nothing in it, [if another] Baheret [the size of] half a bean develops with two [white] hairs in it, it is decided [that he is impure] because they [the Sages] said: If the Baheret preceded the white hairs, it is impure. But if the white hair preceded the Baheret, it is [still] pure. If [a case is] unclear, it is impure. But Rabbi Yehoshua declared [the matter] unsolvable.

בַּהֶרֶת כַּחֲצִי גְרִיס וְאֵין בָּהּ כְּלוּם, נוֹלְדָה בַהֶרֶת כַּחֲצִי גְרִיס וּבָהּ שְׁתֵּי שְׂעָרוֹת, הֲרֵי זוֹ לְהַחְלִיט, מִפְּנֵי שֶׁאָמְרוּ, אִם בַּהֶרֶת קָדְמָה לְשֵׂעָר לָבָן, טָמֵא. וְאִם שֵׂעָר לָבָן קָדַם לְבַהֶרֶת, טָהוֹר. וְאִם סָפֵק, טָמֵא. וְרַבִּי יְהוֹשֻׁעַ קִהָה:

Mishnah Negaim 5:1: Any doubt regarding Negaim [diseased patches on skin, clothes, or houses that create impurity] is considered pure except for this [the following case] and another. And which is [this case]? Someone who had upon him a Baheret [type of diseased patch that creates impurity] like [the size of] a bean, and quarantined it, and behold at the end of the week it was the size of a Sela [coin bigger than a bean, but] it is doubtful whether it was it [the original Nega] or whether another [Nega] had come in place of it, [this case is ruled] impure.

כָּל סְפֵק נְגָעִים טָהוֹר, חוּץ מִזֶּה, וְעוֹד אַחֵר. וְאֵיזֶה, זֶה מִי שֶׁהָיְתָה בוֹ בַּהֶרֶת כַּגְּרִיס וְהִסְגִּירָהּ, בְּסוֹף שָׁבוּעַ וַהֲרֵי הִיא כַּסֶּלַע, סָפֵק שֶׁהִיא הִיא, סָפֵק שֶׁאַחֶרֶת בָּאָה תַחְתֶּיהָ, טָמֵא: