Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 26
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 26, with English translation by Maimonides (Rambam).
שלא להתנבא בשמה, שנאמר "ואשר ידבר, בשם אלוהים אחרים" (דברים יח,כ).
Not to prophesy in the name of false gods, as [Deuteronomy 18:20] states: "[The prophet]... who speaks in the name of other gods [shall die]."