Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 5

וּזְקֵנִים רַבִּים קִבְּלוּ מִיְּהוֹשׁוּעַ, וְקִבַּל עֵלִי מִן הַזְּקֵנִים וּמִפִּינְחָס; וּשְׁמוּאֵל קִבַּל מֵעֵלִי וּבֵית דִּינוֹ, וְדָוִיד קִבַּל מִשְּׁמוּאֵל וּבֵית דִּינוֹ. וַאֲחִיָּה הַשִּׁילוֹנִי, מִיּוֹצְאֵי מִצְרַיִם הָיָה וְלֵוִי הָיָה, וְשָׁמַע מִמֹּשֶׁה, וְהָיָה קָטָן בִּימֵי מֹשֶׁה; וְהוּא קִבַּל מִדָּוִיד וּבֵית דִּינוֹ.

Similarly, throughout his life Joshua taught the Oral Law. Many elders received the tradition from him. (where he outlines some of his deliberations about the composition of the Mishneh Torah), the Rambam writes:I chose to omit the supports and proofs [for the laws], and instead mention the major figures who transmitted the tradition. Thus, I will not say "These are the words of Rabbi ---," or "Rabbi --- says such and such" regarding each particular matter. Instead, I will mention all the sages of the Mishnah and the Talmud, of blessed memory, in general at the beginning of the text. I will state that all the judgments of the Torah - i.e., the Oral Law - were received and transmitted from so and so to so and so, until Ezra and until Moses. I will mention together with [the leading sage of the generation], who received the tradition, the other well-known personalities in his generation whose position in the chain of tradition is equivalent to his. All this [will be done] out of a desire for brevity.The Rambam's willingness to sacrifice the mention of the sources for his decisions in favor of a brief and clear text became a major issue with regard to the acceptance of the Mishneh Torah by other rabbis. The Ra'avad writes:This author abandoned the practice of all the previous authors, who would bring supports for their statements and quote them in the name of their sources. This was of great benefit because, at times, a judge would presume to forbid or permit [something] based on a specific source. If he knew that a greater authority holds a different opinion, he would retract his. However, in this instance, I do not know why I should retract from the tradition I received and my sources because of [the statements] in this work by this author.Afterwards, the Rambam himself regretted his original decision. In a responsum, he wrote that he desired to add the sources on which the decisions of the Mishneh Torah were based. Unfortunately, the Rambam himself never succeeded in composing such a text, and the task of discovering these sources has been left to the sages of subsequent generations. Eli received the tradition from the elders and from Pinchas. Samuel received the tradition from Eli and his court. David received the tradition from Samuel and his court.Achiah of Shiloh was one of those who experienced the exodus from Egypt. 121b. He was a Levite and heard [teachings] from Moses. He was, however, of low stature in Moses' age. Afterwards, he received the tradition from David and his court.