Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 20
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 20, with English translation by Maimonides (Rambam).
שלא ילמד המוסת זכות על המסית, שנאמר "לא תחמול" (דברים יג,ט).
For the person whom a mesit tried to convince not to advance any arguments on behalf of the mesit, as [Deuteronomy 13:9] states: "Do not have pity... [upon him]."