Reference for Bekhorot 50:38

מחלוקת כשבדק ומצא וקמיפלגי בפלוגתא דרבי ור"ש בן גמליאל

For we have learnt, R'Meir used to say: Everything which has a presumption of levitical uncleanness continues for ever in that status,<span class="x" onmousemove="('comment',' If the lost grave is not found, though the whole field had been searched, similarly here, if the blemished animal cannot be identified, according to the first Tanna, all the animals are forbidden.');">¹⁹</span>

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