The 613th mitzvah of the Torah is the obligation for every Jew to write a Torah scroll. In the words of the verse: "And now, write for yourselves this song, and teach it to the Children of Israel. Place it into their mouths, in order that this song will be for Me as a witness for the children of Israel."
Although Rashi and some other commentaries understand this as a command only to write the Torah portion that immediately follows, Ha'azinu (which is referred to as a song), the Talmud understands it as an obligation to write the entire Torah. Maimonides explains that although the word "song" in the verse is referring to Ha'azinu, since it is forbidden to write only a single portion of the Torah on a scroll, it is understood that there is an obligation to write the entire Torah.
Moses fulfilled this commandment by writing (or completing) a Torah scroll on the day he passed away. As the verse states: "And Moses wrote this song on that day, and taught it to the children of Israel… Moses finished writing the words of this Torah in a scroll, until their very completion.”
The mitzvah is echoed in the command for a king to write a “second” Torah scroll. This is understood to mean that in addition to the Torah scroll which he must write as every other Jew does, he also needs to write a second one which he will then carry with him and read from constantly.
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