Musar zu Dewarim 31:26

All of the commandments that were given to Moses from Sinai were given with their interpretation, as it is said, "And I will give thee the tables of stone, and the law (the Torah) and the commandment" (Ex. 24:12). "Torah" refers to the written Torah "and the commandment" refers to the interpretation of the written Torah (Berakoth 5a). And we have been commanded to fulfill the Torah according to the commandment, and this commandment is called the Oral Law. Moses, our teacher, wrote the whole Torah with his own hand, before he died, and he gave a copy to each tribe. And one copy he placed in the Ark, as testimony, as it is said, "Take this book of the law, and put it by the side of the Ark of the covenant of thy Lord your God, that it may be there for witness against thee" (Deut. 31:26).