EXOD927 {Abridged Excerpt from the Rambam's Introduction to Mishneh Torah). All of the mitzvos given to Moshe at Sinai were given with their explanations, as it is written [this verse]: "And I will give you the tablets of stone, the Torah, and the mitzvah." "Torah" refers to the written Law, and " mitzvah" to its explanation (Berachos 5a). And he commanded us to observe the Torah by means of the mitzvah, namely, the Oral Law. Moshe wrote the entire Torah in his own hand before his death and gave a scroll to each tribe. One scroll he placed in the Ark as a testimony (Devarim Rabbah 9:9), as it is written (Devarim 31:26). ... The "mitzvah," which is the explanation of the Torah, he did not write, but he commanded it to the elders and to Yehoshua and all the rest of the Jews, as it is written (ibid.13:1): "Everything that I command you, that you shall observe to do, do not add to it and do not detract from it." Because of this it is called the Oral Law.