Halakhah sa Deuteronomio 17:18: Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll, Mishneh Torah, Positive Mitzvot, Mishneh Torah, Negative Mitzvot & Sefer HaChinukh

A king has the duty of writing for himself another scroll of the Law, in his capacity as sovereign, in addition to the scroll which he owns in his private capacity, as it is said, "And it shall be, when he sitteth upon the throne of his kingdom, that he shall write him a copy of this law, etc." (Deuteronomy 17:18). This copy is corrected by comparison with a scroll kept in the temple court, and under the authority of the Supreme Judicature. The scroll which the king had possessed when he was a private individual, he places in his archives. The one that he wrote or had written for him when he became king, should always be with him. If he goes to war, the scroll is to be with him. When he enters his home, it is to be with him. When he hears a cause (tries a case) it is to be with him. When he sits down to a meal, it is to be placed before him, as it is said, "And it shall be with him, and he shall read therein all the days of his life" (Deuteronomy 17:19).

If he possessed no scroll before he came to the throne, he has to write, after his accession, two scrolls of the law, one of which he places in his archives, while the other is to be always with him. It must never be away from him, except at night, or when he goes to the bath or the lavatory or lies down on his couch to sleep.

That the King shall write a scroll of the Torah for himself, besides the one which every individual should write, so that he shall possess two scrolls of the Torah, as it is said. “And he shall write for himself a copy of this Torah” (Deut. 17:18).