(Menachot 30a) Rabbi Yehoshua bar Abba says that Rav Giddel says that Rav says: One who purchases a Torah scroll in the marketplace is akin to one who snatches a mitzva in the marketplace. If he wrote it himself, the verse ascribes him credit as though he received it at Mount Sinai. And Rav Yehudah says that Rav says: If he emended in it one letter, the verse ascribes him credit as though he wrote it himself. This is because the first individual has the prohibition to have in one's house a sefer which is not corrected; the one who corrects it and makes it fit for reading is as if he wrote it. It is written in chapter HaIsha HaNitarmela [b. Ketubot 19b]: A Torah scroll which is not emended, Rabbi Ami says it is permitted to let it lie for up to thirty days. From that point onwards it is forbidden to let it lie, as it is said [Job 11:14] Do not let injustice reside in your tent. And I say that definitely the greatest mitzvah is to write a Torah scroll, and not to sell it unless to [provide funds to] learn Torah or get married. So too said the Rambam [Laws of the Torah Scroll, ch. 7] that it is a positive commandment incumbent upon every man of Israel to write a Torah scroll for himself, as it is said "And now, write for yourselves this Song," as to say "Write for yourselves a Torah scroll in which is this Song," since we do not write the Torah piecemeal; and this was so in the early generations, when they wrote Torah scrolls and studied from them. But nowadays, when we write Torah scrolls and leave them in synagogues to read from them in public, the positive commandment incumbent upon every man of Israel who can afford it is to write Torah-books and Mishnah and Gemara and commentaries, to make notes in them, he and his offspring. For the commandment to write the Torah is so as to learn it, as it is written [Deuteronomy 31:19] "Teach it to the children of Israel, put it in their mouths." And since by way of the Gemara and its commentaries one understands the commandments and laws in depth, it is these books which one is commanded to write and not sell unless to learn Torah or get married.