Negative commandment · Lo Ta’aseh #363 of 613
For a king not to accumulate many horses
שלא ירבה המלך סוסים
The verse · Devarim 17:16
רַק לֹא־יַרְבֶּה־לּוֹ סוּסִים וְלֹא־יָשִׁיב אֶת־הָעָם מִצְרַיְמָה לְמַעַן הַרְבּוֹת סוּס וַה׳ אָמַר לָכֶם לֹא תֹסִפוּן לָשׁוּב בַּדֶּרֶךְ הַזֶּה עוֹד
“But he shall not multiply horses to himself, nor cause the people to return to Miżrayim, to the end that he should multiply horses: since the Lord has said to you, You shall henceforth return no more that way.”
The Rambam’s words
For a king not to accumulate many horses, as [Deuteronomy 17:16] states: "And he may not accumulate many horses."
שלא ירבה המלך סוסים, שנאמר "לא ירבה לו סוסים" (דברים יז,טז).
Who & when
“And this prohibition is practiced at the time that Israel is in its inhabitation and that we have a king.”
Where in the Mishneh Torah
Hilchot Kings and Wars הלכות מלכים ומלחמות