Reference for Pirkei Avot 2:14

רַבִּי אֶלְעָזָר אוֹמֵר, הֱוֵי שָׁקוּד לִלְמֹד תּוֹרָה, וְדַע מַה שֶּׁתָּשִׁיב לְאֶפִּיקוֹרוֹס. וְדַע לִפְנֵי מִי אַתָּה עָמֵל. וְנֶאֱמָן הוּא בַעַל מְלַאכְתְּךָ שֶׁיְּשַׁלֶּם לָךְ שְׂכַר פְּעֻלָּתֶךְ:

R. Elazar says: Be diligent in learning Torah and know what to reply to an apikores, [from "hefker" (ownerless), one who shames the Torah and regards it as if it were hefker. Or: He makes himself hefker, not pitying his soul, not feeling that evil will come upon him for shaming the Torah or its learners.], and know before whom you toil, [in this reply that you make to the apikores that he not entice you to his beliefs] and your Employer is trusted to pay you for your labor.

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