Reference for Tahorot 9:5

הַמַּנִּיחַ זֵיתִים בַּכּוֹתֵשׁ שֶׁיִּמְתֹּנוּ שֶׁיְּהוּ נוֹחִין לִכְתֹּשׁ, הֲרֵי אֵלּוּ מֻכְשָׁרִים. שֶׁיִּמְתֹּנוּ שֶׁיִּמְלָחֵם, בֵּית שַׁמַּאי אוֹמְרִים, מֻכְשָׁרִים. וּבֵית הִלֵּל אוֹמְרִים, אֵינָן מֻכְשָׁרִים. הַפּוֹצֵעַ זֵיתִים בְּיָדַיִם טְמֵאוֹת, טִמְּאָן:

One who leaves olives in the press for them to soften to be easier to press, they are thereby primed [for impurity, by the liquid that emerges from them]. If [one left olives in a press] for them to soften to be salted, Beit Shammai says: they are primed [for impurity]; and Beit Hillel says: they are not primed. One who crushes olives with impure hands has rendered them impure.

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