Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 38
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 38, with English translation by Maimonides (Rambam).
שלא לשאול בחלום מן המתים, שנאמר "ודורש אל המתים" (דברים יח,יא).
Not to seek information from the dead in dreams, as [Deuteronomy, ibid.] states: "[There shall not be found among you...] one who attempts to communicate with the dead."