Reference for Bekhorot 52:12

אמר רבי אלעזר אמר ריש לקיש

- Said R'Ela: Resh Lakish holds [that the reason is] because it is impossible for wool to he free from loosely connected threads.<span class="x" onmousemove="('comment',' For it is a usual thing, and, consequently, if we adopted R. Nathan's interpretation, there would scarcely be any wool that would be allowed to be used in such circumstances.');">⁶</span>

Jastrow