Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 364
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 364, with English translation by Maimonides (Rambam).
שלא ירבה המלך נשים, שנאמר "לא ירבה לו נשים" (דברים יז,יז).
For a king not to accumulate many wives, as [Deuteronomy 17:17] states: "And he may not accumulate many wives."