Our Rabbis tell us that Moshe Rabeinu gave us the previous four parts of the Torah, “mipi hagvura,” from G-d’s mouth, and the fifth and final part of the Torah, “mipi atzmo,” from himself.
The meaning of this is as follows:
From G-d’s mouth, so to speak, means that G-d transmitted the information to Moshe Rabeinu and Moshe Rabeinu put it in writing. The writing was free of any of Moshe Rabeinu’s perspective, it was a direct transmission.
From Moshe Rabeinu himself means that the G-dly information became integrated in Moshe Rabeinu’s mind. In other words, the fifth book came from what Moshe Rabeinu internalized and transmitted to us from himself.
Likkutei Sichos vol. 36, page 42, see footnote 49