Midrash su Deuteronomio 17:19

1) (Vayikra 4:22): "If (asher) a leader (nassi) sin ["asher," similar to "ashrei" ("fortunate")]: R. Yochanan b. Zakkai said: Fortunate is the generation whose nassi brings a sin-offering for his unwitting sin. If he brings a sin-offering for his unwitting sin, how much more so (is he contrite over) his deliberate sin. And if his nassi brings a sin-offering, how much more so is he (the common man) moved to bring a sin-offering (for his sin)! "a nassi": I might think the nassi of a tribe, like Nachshon; it is, therefore, written: "And he do one of all the mitzvoth of the L–rd his G d," and, elsewhere, (in respect to a king) (Devarim 17:19): "so that he may learn to fear the L–rd his G d." Just as there, the nassi is one who has no one above him but the L–rd his G d, so, here. (Scripture speaks of) a nassi who has no one over him but the L–rd his G d (i.e., a king).

MISHNAH: The king must write the Holy Scrolls for himself; when he goes to war he must bear them with him; when he enters the city they must be with him; and the same when he sits judging the people; and when he takes his meals they must be placed opposite him, as it is said (Deut. 17, 19) And it shall be with him, and he shall read therein all the days of his life.

GEMARA: Rabba said: "Even if his ancestors left one a scroll, nevertlieless it is a meritorious act for him to write the Holy Scrolls at his own expense, as it is said (Deut. 31, 19) Now, therefore, write this song." Abaye objected [from our Mishnah]: "He shall write the Holy Scrolls for himself," and must not be satisfied with those of his parents. And this speaks only of a king, but not of an ordinary man. Our Mishnah treats of two Holy Scrolls, as we are taught in the following Baraitha (Ib. 17, 18) He shall write for himself a copy of this law — i.e., He must write for himself two Holy Scrolls, one which he must carry wherever he goes, and one which shall remain in his treasury. The one he has to bear with him he shall write in the form of an amulet, and place it on his arm, as it is said (Ps. 16, 8) I have always set the Lord before me. However, he must not enter a bath or a toilet with it, as it is said (Ib., ib. 19) And it shall be with him and he shall read; i.e., it shall be with him in those places where it is permitted to be read, but not in those places where it is not permitted.