Mishnayos Keilim Perek 27 Mishnah 8

Similarly, if a piece of cloth was woven to the size of three [fingerbreadths] square, and it contracted corpse uncleanness, and afterwards he finished the entire piece, and then he removed a single thread from its original part, it is free from corpse uncleanness but is still unclean from contact with corpse uncleanness. If a thread was removed from the original part and then all the cloth was finished it remains clean, for the sages have ruled: if a piece of three [fingerbreadths] square is lessened it becomes clean. But if one of three handbreadths square is lessened, even though it is released from midras, it is still susceptible to all other forms of uncleanness.

וְכֵן בֶּגֶד שֶׁאָרַג בּוֹ שָׁלֹשׁ עַל שָׁלֹשׁ, 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, שָׁלֹשׁ עַל שָׁלֹשׁ שֶׁנִּתְמַעֵט, טָהוֹר. אֲבָל שְׁלֹשָׁה עַל שְׁלֹשָׁה שֶׁנִּתְמַעֵט, אַף עַל פִּי שֶׁטָּהוֹר מִמִּדְרָס, טָמֵא בְכָל הַטֻּמְאוֹת: