Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 37
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 37, with English translation by Maimonides (Rambam).
שלא לשאול ביידעוני, שנאמר "ושואל אוב ויידעוני" (דברים יח,יא).
Not to consult a yid'oni, as [Deuteronomy, ibid.] states: "[There shall not be found among you...] one who consults an ov or a yid'oni."