Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 363
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 363, with English translation by Maimonides (Rambam).
שלא ירבה המלך סוסים, שנאמר "לא ירבה לו סוסים" (דברים יז,טז).
For a king not to accumulate many horses, as [Deuteronomy 17:16] states: "And he may not accumulate many horses."