Midrash Rabbah Shemot 46:1 (hebrewbooks link)
...הִתְחִיל מִצְטַעֵר עַל שִׁבּוּר הַלּוּחוֹת, וְאָמַר לוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא אַל תִּצְטָעֵר בַּלּוּחוֹת הָרִאשׁוֹנוֹת, שֶׁלֹא הָיוּ אֶלָּא עֲשֶׂרֶת הַדִּבְּרוֹת לְבָד, וּבַלּוּחוֹת הַשְּׁנִיִּים אֲנִי נוֹתֵן לְךָ שֶׁיְהֵא בָהֶם הֲלָכוֹת מִדְרָשׁ וְאַגָּדוֹת, הֲדָא הוּא דִכְתִיב: וְיַגֶּד לְךָ תַּעֲלֻמוֹת חָכְמָה כִּי כִפְלַיִם לְתוּשִׁיָּה,
...He started feeling bad that he broke the tablets, G-d told him: Do not feel bad about the first tablets, for they only contained the ten commandments, however in the second tablets i will give you, that they will have Halcaha Midrash and Agadah, this is what is said: (Job 11): I will tell you hidden wisdom for it shall be double comforting
See here for an explanation from the Lubavitcher Rebbe why the first Luchos needed to be broken before the Oral Torah could be given.
