Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 19
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 19, with English translation by Maimonides (Rambam).
שלא להציל המסית אלא עומד על דמו, שנאמר "לא תחוס עינך עליו" (דברים יג,ט).
Not to [try to] save a mesit, but rather to see to it that he is executed, as [Deuteronomy 13:9] states: "Do not have mercy upon him."