Reference for Kelim 28:1

שָׁלֹשׁ עַל שָׁלֹשׁ שֶׁנְּתָנָהּ בְּכַדּוּר, אוֹ שֶׁעֲשָׂאָהּ כַּדּוּר בִּפְנֵי עַצְמָהּ, טְהוֹרָה. אֲבָל שְׁלֹשָׁה עַל שְׁלֹשָׁה שֶׁנְּתָנוֹ בְכַדּוּר, טָמֵא. עֲשָׂאוֹ כַדּוּר בִּפְנֵי עַצְמוֹ, טָהוֹר, מִפְּנֵי שֶׁהַתֶּפֶר מְמַעֲטוֹ:

A [piece of cloth] three by three [fingerbreadths] which was stuffed into a ball, or was itself made into a ball, is pure. But a [piece of cloth] three by three [handbreadths] which was stuffed into a ball is [susceptible to being rendered] impure. If it was itself made it into a ball, it is pure, because the sewing diminishes it.

Jastrow