The Talmud states: 'The words spoken orally (divrei Torah shebaal peh) you are not permitted to say in writing.' It also records the teaching that God made a covenant with Israel specifically on account of the Oral Torah, deriving this from Exodus 34:27.
אָמַר רַבִּי אֶלְעָזָר: תּוֹרָה – רוֹב בִּכְתָב וּמִיעוּט עַל פֶּה, שֶׁנֶּאֱמַר: ״אֶכְתּוֹב לוֹ רוּבֵּי תּוֹרָתִי כְּמוֹ זָר נֶחְשָׁבוּ״. וְרַבִּי יוֹחָנָן אָמַר: רוֹב עַל פֶּה וּמִיעוּט בִּכְתָב, שֶׁנֶּאֱמַר: ״כִּי עַל פִּי הַדְּבָרִים הָאֵלֶּה״. וְאִידַּךְ נָמֵי, הָכְתִיב: ״אֶכְתּוֹב לוֹ רוּבֵּי תּוֹרָתִי״! הָהוּא, אַתְמוֹהֵי קָא מַתְמַהּ: אֶכְתּוֹב לוֹ רוּבֵּי תּוֹרָתִי?! הֲלֹא ״כְּמוֹ זָר נֶחְשָׁבוּ״! אָמַר רַבִּי יוֹחָנָן: לֹא כָּרַת הַקָּדוֹשׁ בָּרוּךְ הוּא בְּרִית עִם יִשְׂרָאֵל, אֶלָּא בִּשְׁבִיל דְּבָרִים שֶׁבְּעַל פֶּה – שֶׁנֶּאֱמַר: ״כִּי עַל פִּי הַדְּבָרִים הָאֵלֶּה כָּרַתִּי אִתְּךָ בְּרִית וְאֶת יִשְׂרָאֵל״.
§ The Gemara continues its discussion concerning the writing of the Torah: Rabbi Elazar says: The majority of the Torah was transmitted in writing, while the minority was transmitted orally, as it is stated: “I wrote for him the greater part of My Torah; they were reckoned a strange thing” (Hosea 8:12), meaning that the majority of the Torah was transmitted in written form. And Rabbi Yoḥanan says: The majority of the Torah was transmitted orally [al peh], while the minority was transmitted in writing, as it is stated with regard to the giving of the Torah to Moses on Mount Sinai: “For on the basis of [al pi] these matters I have made a covenant with you and with Israel” (Exodus 34:27), which indicates that the greater part of the Sinaitic covenant was taught orally. The Gemara asks: And according to the other Sage, Rabbi Yoḥanan, as well, isn’t it written: “I wrote for him the greater part of My Torah”? How does he understand this verse? The Gemara answers: This verse is not a statement, but rather a rhetorical question expressing bewilderment: For did I write for him the greater part of My Torah? In that case they, the Jewish people, would be reckoned as strangers, meaning that there would be no difference between them and the nations of the world if everything was written down. Rather, the majority of the Torah must remain an oral tradition. Rabbi Yoḥanan says: The Holy One, Blessed be He, made a covenant with the Jewish people only for the sake of the matters that were transmitted orally [be’al peh], as it is stated: “For on the basis of [al pi] these matters I have made a covenant with you and with Israel” (Exodus 34:27).