Deuteronomy 17:15-18

15Thou shalt in any wise set him king over thee, whom Yhwh thy God shall choose: one from among thy brethren shalt thou set king over thee: thou mayest not set a stranger over thee, which is not thy brother.

16But he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he should multiply horses: forasmuch as Yhwh hath said unto you, Ye shall henceforth return no more that way.

17Neither shall he multiply wives to himself, that his heart turn not away: neither shall he greatly multiply to himself silver and gold.

18And it shall be, when he sitteth upon the throne of his kingdom, that he shall write him a copy of this law in a book out of that which is before the priests the Levites: