Reference for Niddah 56:11

רבי עקיבא אומר

Now did not R. Eliezer state there<span class="x" onmousemove="('comment',' Shebu. 18b. ');">¹⁸</span> that 'it is necessary that a person should know whether he contracted uncleanness through a creeping thing or the carcass of a beast'?<span class="x" onmousemove="('comment',' Of course he did. ');">¹⁹</span>

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