Reference for Pesachim 31:1

ספק משקין ליטמא טמא לטמא אחרים טהור דברי ר"מ וכן היה רבי אלעזר אומר כדבריו רבי יהודה אומר לכל טמא

Doubtful [cases of uncleanness with] fluids,<span class="x" onmousemove="('comment',' E.g. if an unclean person. whose touch defiles liquids, puts his hand into a vessel, and it is not known whether he actually touched the liquid there or not.');">¹</span> in respect of becoming unclean themselves, are unclean; in respect of defiling others,<span class="x" onmousemove="('comment',' E.g., if unclean liquid fell near food and it is unknown whether it actually touched it or not.');">²</span>

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