Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 36
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 36, with English translation by Maimonides (Rambam).
שלא לשאול באוב, שנאמר "ושואל אוב" (דברים יח,יא).
Not to consult an ov, as [Deuteronomy, ibid.] states: "[There shall not be found among you...] one who consults an ov."