Halakhah zu Dewarim 17:18

That the king write a second Torah scroll for himself: That it is a commandment on the king that will be over Israel to write for himself a specific scroll of the Torah, from the angle of [his being king], that it be with him always and [from] which he read - besides the other Torah scroll which is a commandment for him, like every one of Israel, to write, as we will write in the last commandment of the book, with God's help (Sefer HaChinukh 613). And about this is it stated (Deuteronomy 17:18), "and he shall write for himself a copy of this Torah, etc."