Reference for Bekhorot 50:1

וחכמים אוסרין דברי ר' יהודה

WHEREAS THE SAGES DECLARE IT FORBIDDEN.<span class="x" onmousemove="('comment',' For if you permit the use of wool plucked when the animal is alive, one may be led to detain the firstling in order to benefit from its wool, and this may eventually bring about a breaking of the law with reference to working and shearing.');">¹</span>

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