Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 27
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 27, with English translation by Maimonides (Rambam).
שלא להתנבא בשקר, שנאמר "אשר יזיד לדבר דבר בשמי, את אשר לא ציוויתיו" (דברים יח,כ).
Not to relate false prophecies, as [Deuteronomy 18:20] states: "When a prophet presumptuously makes a declaration in My name which I have not commanded him...."