Deuteronomy 17:14-20

After you've entered the land that the Lord your God is giving you, and have taken it over and settled in it, and you decide, “Let's have a king to rule over us like all the other nations around us do,” you can have a king but only one chosen by the Lord your God. He must be an Israelite. You must not have a king who is a foreigner; someone who is not an Israelite. Your king must not have large numbers of horses, or send his men to Egypt to buy more horses, because the Lord has declared, “You must never return there again.” He must not have many wives, so they don't lead him away from following the Lord. He must not have large quantities of silver and gold. Once he is king and sits on his royal throne, he must make a copy for himself of these instructions, writing them on a scroll in the presence of the Levitical priests. He is to keep it with him, and he is to read from it each day throughout his life, so he may learn to respect the Lord his God by being careful to follow every word of these instructions and regulations. Then he won't think more of himself than his fellow Israelites, and he won't deviate from the commandments, so that he and his sons may have long reigns over the kingdom of Israel.