Reference for Bekhorot 51:6

אמר רבי יהודה

If one plucks wool from an unblemished firstling, although subsequently there appeared a blemish on it and he slaughtered it, the wool is forbidden [to be used].<span class="x" onmousemove="('comment',' For in that case, even Akabya agrees that the wool is forbidden to be used, for we apprehend that he may be led to detain the firstling and prevent it from being offered up on the altar. The same ruling also applies to wool which has become detached from the animal.');">⁵</span>

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