"And they sacrificed to it and said" (Exodus 32:8). Rav Nachman said in the name of Rabbah bar Avuha, who said in the name of Rav: Since he said to it, "You are my god," if he did so inadvertently he is liable to bring an offering, according to Rabbi Akiva. But this is obvious; this is the same as a blasphemer! You might have thought that Rabbi Akiva holds one liable only in the case of a blasphemer, concerning whom excision (karet) is written, but here, where excision is not written, you might say he is not liable. Therefore it teaches us that they are juxtaposed, one to the other, as it is written, "And they sacrificed to it and said, These are your gods, O Israel" (Exodus 32:8).
Why the Calf Worshipper Owes an Offering Like a Blasphemer
Yalkut Shimoni on Torah 391:18
וַיִּזְבְּחוּ לוֹ וַיֹּאמְרוּ. אֲמַר רַב נַחְמָן אֲמַר רַבָּה בַּר (אַבָּהוּ) [אָבוּהָ] אֲמַר רַב. כֵּיוַן שֶׁאָמַר לוֹ אֵלִי אַתָּה, בְּשׁוֹגֵג, חַיָּב קָרְבָּן לְרַבִּי עֲקִיבָא, פְּשִׁיטָא, הַיְינוּ מְגַדֵּף מַהוּ דְתֵימָא, עַד כָּאן לָא מְחַיֵּב רַבִּי עֲקִיבָא אֶלָּא בִּמְגַדֵּף, דִּכְתִיב בֵּיהּ כָּרֵת, אֲבָל הָכָא דְּלָא כְתִיב כָּרֵת אֵימָא לָא, קָא מַשְׁמַע לָן אִיתְקוּשֵׁי אִיתְקוּשׁ, דִּכְתִיב וַיִּזְבְּחוּ לוֹ וַיֹּאמְרוּ אֵלֶּה אֱלֹהֶיךָ יִשְׂרָאֵל.