Moshe Rabbeinu said the book of Devorim from his own accord to the Jewish people. Our Rabbis tell us, that this book is, “mipi atzmo,” from Moshe himself. Moshe Rabbeinu internalized Hashem’s transmission so well, that the Torah he gave over was also Torah and included in the Five Books of Moses.
From the second portion of the parsha, we see that Moshe Rabbeinu attributed the appointment of leader and judges to himself. However, in parshas Yisro, we see that it was Yisro’s idea to establish a delegation. It follows that Moshe Rabbeinu’s internalization of Torah (parshas Yisro is part of Torah), that it became “his” idea.
Likkutei Sichos vol. 16, page 209