Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 17
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 17, with English translation by Maimonides (Rambam).
שלא לאהוב המסית, שנאמר "לא תאבה לו" (דברים יג,ט).
Not to show affection for a mesit, as [Deuteronomy 13:9] states: "Do not show appreciation for him."