Reference for Chullin 63:1

נשחטה בהמה אחרת עמה לרבי נתן פרה פסולה בהמה כשרה

If another animal was [accidentally] slaughtered with it, according to R'Nathan, the Red Cow is invalid<span class="x" onmousemove="('comment',' Even though his mind was not taken away from the slaughtering of the Red Cow it is invalid; because it is written: Num. XIX, 3. And he shall slaughter it, which means, 'it by itself', and not another animal with it.');">¹</span>

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