Deuteronomy 17:15-18

15thou shalt surely set him king over thee, whom Jehovah thy God shall choose: one from among thy brethren shalt thou set king over thee; thou mayest not put a foreigner over thee, who is not thy brother. 16Only he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he may multiply horses; forasmuch as Jehovah 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: