Deuteronomy 17:15-18

15you shall most certainly set a king over you whom the Lord your God chooses. You shall set a king over you from among your countrymen (brothers); you may not choose a foreigner [to rule] over you who is not your countryman.

16Further, he shall not acquire many [war] horses for himself, nor make the people return to Egypt in order to acquire horses [to expand his military power], since the Lord said to you, ‘You shall never return that way again.’

17He shall not acquire multiple wives for himself, or else his heart will turn away [from God]; nor [for the same reason] shall he acquire great amounts of silver and gold.

18“Now it shall come about when he sits on the throne of his kingdom, he shall write for himself a copy of this law on a scroll in the presence of the Levitical priests.