Rashi on Deuteronomy 17:18/רש"י על דברים י״ז:י״ח

1והיה כשבתו AND IT SHALL BE WHEN HE SITTETH [UPON THE THRONE OF HIS KINGDOM] — if he acts thus (as prescribed in the previous verses) he is worthy that his kingdom should endure (Sifrei Devarim 160:1).

2את משנה התורה [HE SHALL WRITE HIM] A משנה תורה — i.e. two scrolls of the Law, one that is placed in his treasury and the other that goes out and comes in with him (i.e. a small scroll which he carries everywhere with him) (Sanhedrin 21b). Onkelos, however, renders משנה by פתשגן, a copy; he thus interpreted the word משנה in the sense of שנן, repeating and uttering.