Reference for Negaim 4:4
שְׁתֵּי שְׂעָרוֹת, עִקָּרָן מַשְׁחִיר וְרֹאשָׁן מַלְבִּין, טָהוֹר. עִקָּרָן מַלְבִּין וְרֹאשָׁן מַשְׁחִיר, טָמֵא. כַּמָּה יְהֵא בַלַּבְנוּנִית, רַבִּי מֵאִיר אוֹמֵר, כָּל שֶׁהוּא. רַבִּי שִׁמְעוֹן אוֹמֵר, כְּדֵי לִקְרֹץ בְּזוּג. הָיְתָה אַחַת מִלְּמַטָּה וְנֶחְלְקָה מִלְמַעְלָה וְהִיא נִרְאֵית כִּשְׁתַּיִם, טָהוֹר. בַּהֶרֶת וּבוֹ שֵׂעָר לָבָן אוֹ שֵׂעָר שָׁחוֹר, טָמֵא. אֵין חוֹשְׁשִׁין שֶׁמָּא מִעֵט מְקוֹם שֵׂעָר שָׁחוֹר אֶת הַבַּהֶרֶת, מִפְּנֵי שֶׁאֵין בּוֹ מַמָּשׁ:
Two hairs whose roots are black and tips are white are [still] pure. [Two hairs] whose roots are white and tips are black are impure. How much whiteness [is required]? Rabbi Meir says: Any amount. Rabbi Shimon says: [Enough] to be able to cut [them] with scissors. If [the hair] was one at the bottom but it split at the top and looks like two, it is [still] pure. A <i>Baheret</i> [type of diseased patch that creates impurity] in which there is white hair or black hair is impure, [because] we do not worry lest the area of the black hair reduced the [total area of the] <i>Baheret</i> [and made it smaller than the minimum size] because it does not have significance.