Reference for Bekhorot 33:68

הדלה פשתן על גבי היגא ה"נ דלישתני

AND THE SECOND ONE [IN THE ISRAELITE'S POSSESSION] IS LEFT TO PASTURE UNTIL IT BECOMES BLEMISHED.<span class="x" onmousemove="('comment',' And after that he eats it, the reason being that it is a doubtful first-born and cannot therefore, be eaten unblemished. The same ruling applies also to the priest's animal, as we are dealing with firstlings in our day, after the destruction of the Temple. Or, indeed, it may, even deal with a firstling in Temple times; seeing that there is uncertainty as to whether the animal is a firstling, it cannot be killed in the Temple court.');">³⁷</span>

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