Reference for Zevachim 187:5

אין טעון כיבוס כו': מנה"מ דת"ר יכול נתז על מקצת בגד יהא כל הבגד טעון כיבוס ת"ל (ויקרא ו, כ) אשר יזה לא אמרתי לך אלא מקום דם בלבד:

Raba said: They disagree about an [untrimmed] hide which he intended to trim.<span class="x" onmousemove="('comment',' 'Uzba is anything used as a rug or mat or tablecloth; it is generally of hide, but sometimes of cloth. Now, if one intended to use it for such purpose, it immediately ranks as a utensil, even before it is trimmed, and hence can be defiled. But if he intended trimming it, it cannot become unclean until he either trims it or abandons his intention.');">⁸</span> He who maintains [that it must be] eligible, this too is eligible; he however who maintains, anything which can contract uncleanness, this however cannot contract uncleanness until he trims it. And it was taught even so: R'Simeon B'Menassia said: A hide which [its owner] intended trimming is clean<span class="x" onmousemove="('comment',' I.e., it cannot become unclean.');">⁹</span>

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