Deuteronomy 17:16-17 -

Certain rules are prescribed for the king. It is forbidden to him to multiply horses, to multiply wives, and to amass large treasures of silver and gold, and he must have a copy of the Law written out for him from that kept by the priests, that he might have it by him, and read it all the days of his life. The multiplying of horses is prohibited, because this would bring Israel into intercourse and friendly relations with Egypt, and might tend to their going back to that country from which they had been so marvelously delivered; a prohibition which could only have been given at an early stage in the history of the people, for at a later period, after they had been well established in Canaan, such a prohibition for such a reason would have been simply ridiculous. The prohibition to multiply wives and to amass large treasures has respect to the usage common from the earliest period with Oriental monarchs to have vast harems and huge accumulations of the precious metals, as much for ostentation as for either luxury or use; and as there was no small danger of the King of Israel being seduced to follow this usage, and so to have his heart turned away from the Lord, it was fitting that such a prohibition should be prospectively enacted for his guidance. Both these prohibitions were neglected by Solomon, and probably by others of the Jewish kings; but this only indicates that the law was so ancient that it had come in their time to be regarded as obsolete. The rule that the king was to write him a copy of the Law for his own constant use does not necessarily imply that he was to write this with his own hand; he might cause it to be written by some qualified scribe for him.