Deuteronomy 17:17

Neither shall he multiply wives to himself As the manner of other kings was, contrary to the design of God from the beginning. That his heart turn not away From God and his law, as Solomon's did. Neither silver nor gold Lest this should lift up his heart in confidence and pride, which God abhors, and beget in him a contempt of his people.

They are not simply forbidden to be rich, if God made them so, which was the case of David, Solomon, Jehoshaphat, and some others; but they are forbidden, either inordinately to desire, or irregularly to procure great riches, by grinding the faces of their own subjects, or possessing themselves, contrary to justice, of the property of others.