Halakhah for Sanhedrin 42:19

That is that He commanded us that every man among us should write a Torah scroll for himself. And if he writes it with his hand, Scripture accounts it to him as if he received it at Mount Sinai. But if it is impossible for him to write it, he should buy it or hire someone to write it for him. And this is His saying, "write for yourselves this song " (Deuteronomy 31:19). And it is not permitted to write [only] sections. For when He said, "this song," He intended the whole Torah, which includes this song (of Haazinu). And the language of the Gemara (Sanhedrin 21b) is, "Rabbah said, 'Even if his ancestors left him a Torah scroll, it is a commandment to write a scroll of his own, as it is stated, "write for yourselves."' Abaye raised an objection to him (from a baraita concerning the king’s Torah scroll), '"And he writes himself a Torah scroll for himself, so that he does not become proud from the Torah scroll of his ancestors." A king, yes, but an ordinary person, no!'" And the answer was, "No, [the ruling of that baraita] was only necessary [to teach that the king is commanded to write] two Torahs; as it is taught in a baraita, 'And he must write two Torahs for himself.'" This means to say that the difference between a king and an ordinary person is that every man is obligated to write a Torah scroll, whereas a king, two - as it is explained in the second chapter of Sanhedrin. And the regulations of this commandment - meaning to say, the writing of a Torah scroll and its stipulations - have already been explained in the third chapter of Menachot and in Shabbat, Chapter 16. (See Parashat Nitzavim; Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll.)