Reference for Pesachim 38:9

ולטעמיך אימא סיפא דרך עליה טהורין הא דעלמא טמאין

R'Jose son R'Abin said: E.g. if the cow was muzzled and came from without Jerusalem.<span class="x" onmousemove="('comment',' Hence it must have swallowed it outside, where a doubtfully unclean utensil is unclean, and it remains so even when it enters Jerusalem.');">¹²</span>

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