Reference for Bekhorot 40:12

הלך אחר רוב בהמות ורוב בהמות אין חולבות אלא אם כן יולדות

- Said R'Hanina of Sura: The difference between them is whether milk exempts [from the law of the firstling].<span class="x" onmousemove="('comment',' If it gives milk in its first year, or if it was born in the Israelite's house and we did not see it giving birth until after the first year, but meanwh it gave milk.');">⁶</span>

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