Reference for Chullin 232:15

אינהו דלא בדילי מינייהו וכי חזו להו לא מפרשי להו גזרו בהו רבנן

we separate them, the Rabbis imposed no restriction as a precaution; with regard to theirs, however, since they do not keep away from unclean animals, and whenever they see them together<span class="x" onmousemove="('comment',' I.e., clean animals with unclean animals.');">⁷</span>

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