Talmud for Bava Metzia 66:2

מאי דרוש כדדריש רבי יהודה ברבי אלעאי מאי דכתיב (ישעיהו נח, א) הגד לעמי פשעם ולבית יעקב חטאתם

was taught in the days of Rabbi; thereupon everyone forsook the Mishnah and went to the Gemara; hence he subsequently taught them, 'Yet run always to the Mishnah more than to the Gemara.'<span class="x" onmousemove="('comment',' The two are not really in opposition. The Mishnah itself needs full discussion (Gemara) before it can be intelligently understood; on the other hand, discussion cannot be profitable unless it takes the Mishnah as its basis. It would appear that when Gemara was praised, number of disciples eagerly applied themselves thereto, forgetting however that the Mishnah is the foundation; and therefore the new statement was made, which is not so much a new statement as a fuller explanation of the old. — It is noteworthy that Gemara, i.e., discussion on the Mishnah, was already rife in the days of Rabbi (i.e.. R. Judah the Prince c. first half of third century C.E.); cf. Weiss, Dor II, p. 209. ');">²</span>

Jerusalem Talmud Bava Metzia