Keilim27: 8
וְכֵן בֶּגֶד שֶׁאָרַג בּוֹ שָׁלשׁ עַל שָׁלשׁ, וְנִטְמָא טְמֵא מֵת, וְהִשְׁלִים עָלָיו אֶת כָּל הַבֶּגֶד, וְאַחַר כָּךְ נָטַל חוּט אֶחָד מִתְּחִלָּתוֹ — טָהוֹר מִטְּמֵא מֵת, אֲבָל טָמֵא מַגַּע טְמֵא מֵת. נָטַל חוּט אֶחָד מִתְּחִלָּתוֹ, וְאַחַר כָּךְ הִשְׁלִים עָלָיו אֶת כָּל הַבֶּגֶד — טָהוֹר. מִפְּנֵי שֶׁאָמְרוּ: שָׁלשׁ עַל שָׁלשׁ שֶׁנִּתְמַעֵט — טָהוֹר, אֲבָל שְׁלשָׁה עַל שְׁלשָׁה שֶׁנִּתְמַעֵט, אַף עַל פִּי שֶׁטָּהוֹר מִמִּדְרָס — טָמֵא בְכָל הַטֻּמְאוֹת.
Similarly, [if a piece of] cloth that was woven [to the size] of three [fingerbreadths] by three [fingerbreadths] contracted corpse-tumah, and [subsequently] one finished the whole [piece of] cloth, and afterward one thread was removed from the original [cloth], it becomes tahor from corpse-tumah; but it is tamei [due to] contact with a t’mei meis. [If] one thread was removed from the original [cloth], and subsequently one finished the whole [piece of] cloth, it is tahor. Because [the Sages] said: [If a piece measuring] three [fingerbreadths] by three [fingerbreadths] is lessened, it is tahor. But [if] a piece measuring] three [handbreadths] by three [handbreadths] is lessened, even though it is not susceptible to midras tumah, it is [still] susceptible to all other forms of tumah.