Negative commandment · Lo Ta’aseh #365 of 613
For a king not to accumulate much silver and gold
שלא ירבה לו המלך כסף וזהב
The verse · Devarim 17:17
וְלֹא יַרְבֶּה־לּוֹ נָשִׁים וְלֹא יָסוּר לְבָבוֹ וְכֶסֶף וְזָהָב לֹא יַרְבֶּה־לּוֹ מְאֹד
“Neither shall he multiply wives to himself, that his heart turn not away: neither shall he greatly multiply to himself silver and gold.”
The Rambam’s words
For a king not to accumulate much silver and gold, as [Deuteronomy, ibid.] continues: "And he may not accumulate very much silver and gold."
שלא ירבה לו המלך כסף וזהב, שנאמר "וכסף וזהב, לא ירבה לו מאוד" (דברים יז,יז).
Why · Sefer HaChinuch #502
That the king not amass much silver and gold for himself
“From the roots of the commandment is that which is elucidated in Scripture, “That his heart not get haughty, etc.” (Deuteronomy 17:20).”
Where in the Mishneh Torah
Hilchot Kings and Wars הלכות מלכים ומלחמות