Reference for Chullin 9:3
הַמַּפְשִׁיט בַּבְּהֵמָה וּבַחַיָּה, בַּטְּהוֹרָה וּבַטְּמֵאָה, בַּדַּקָּה וּבַגַּסָּה, לְשָׁטִיחַ, כְּדֵי אֲחִיזָה. וּלְחֵמֶת, עַד שֶׁיַּפְשִׁיט אֶת הֶחָזֶה. הַמַּרְגִּיל, כֻּלּוֹ חִבּוּר לַטֻּמְאָה, לִטָּמֵא וּלְטַמֵּא. עוֹר שֶׁעַל הַצַּוָּאר, רַבִּי יוֹחָנָן בֶּן נוּרִי אוֹמֵר, אֵינוֹ חִבּוּר. וַחֲכָמִים אוֹמְרִים, חִבּוּר, עַד שֶׁיַּפְשִׁיט אֶת כֻּלּוֹ:
When a person removes the skin of a domestic or wild animal, whether clean or unclean, large or small, in order to cover himself therewith, pollution is contracted and communicated when as much skin is removed as can be taken hold of, and if to make a bottle of skin, until the skin over the breast is removed. If the skinning was commenced from the legs, the whole is considered as connecting, and subject to contract and communicate pollution. The skin covering the neck is not considered as connecting by R. Jochanan ben Nouri, but the sages do so consider it until the whole skin is removed.