Reference for Bekhorot 106:2

מותר לו לאדם להטיל מום בבכור קודם שיצא לאויר העולם

For Rab Judah said: One may maim a first-born before it is born.<span class="x" onmousemove="('comment',' Lit., 'issues into the air of the world'; this remedy being even a better one than that of a heathen sharing a part of the animal, v. supra 3b. Therefore a first-born is holy because we do not entertain a fear lest one might be led to commit an offence, seeing that he could, if he wished, eliminate all sanctity from the animal at the outset.');">¹</span>

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