Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 172
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 172, with English translation by Maimonides (Rambam).
לשמוע מכל נביא שיהיה בכל דור ודור אם לא יוסיף ולא יגרע, שנאמר: "אליו, תשמעון" (דברים יח,טו).
To listen to any prophet who will arise in any era, provided he does not add or detract [from the Torah's commandments], as [Deuteronomy 18:15] states: "You shall listen to him."