Reference for Chullin 259:8

חוץ מן הבכור ומן המעשר

ARE SUBJECT TO THE LAW OF THE FIRSTLING<span class="x" onmousemove="('comment',' And if they now bear a male firstling, even though the animal became pregnant before redemption, it belongs to the priest; cf. Num. XVIII, 15-28.');">⁷</span>

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