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On page 179 of the Artscroll Siddur (1984 ISBN: 0-89906-650-X) it lists “The Thirteen Principles of Faith”. Number eight stats “…that the Torah now in our hands is the same one that was given to Moses…”. In the comments it stats “Every word in the Torah was dictated to Moses by God… Moreover, the same applies to the Oral Law that explains the Torah. All was given by God to Moses”.
In a previous question the Rambam is quoted as saying the following.
... Similarly, the transmitted explanation of the Torah was also received from G-d. This is what we do nowadays with the forms of the sukah, lulav, shofar, tzitzis, tefilin, etc., which are the exact same forms that G-d gave to Mosheh. ...
In the above referenced question a comment is made about how it all boils down to what "...transmitted explanation..." means. This commenter then states that some consider that to be the source from which the Mishnah was redacted to all of the modern Talmud.
Are those beliefs (source of Mishnah, all of Talmud, somewhere between) based on any source material, or just what someone happens to like to believe?
If they are based on something does anyone know of the proof text for those beliefs?