Talmud for Nedarim 11:7
יוֹדֵעַ אֲנִי שֶׁיֵּשׁ נְדָרִים, אֲבָל אֵינִי יוֹדֵעַ שֶׁיֵּשׁ מְפֵרִין, יָפֵר. יוֹדֵעַ אֲנִי שֶׁיֵּשׁ מְפֵרִין אֲבָל אֵינִי יוֹדֵעַ שֶׁזֶּה נֶדֶר, רַבִּי מֵאִיר אוֹמֵר, לֹא יָפֵר, וַחֲכָמִים אוֹמְרִים, יָפֵר:
(If he said:) "I knew that there were vows, but I did not know that there were annulments" [i.e., that he had the authority to annul them], he may annul them [on the day he learns that he has the authority to do so, that day being like "the day he hears it."] But (if he said:) "I did not know that it was a vow," [and that it needed annulment], R. Meir says: He may not annul it. [For since he knew that he could annul it, even though he did not know it was a vow, he should have annulled it in any event. The halachah is not in accordance with R. Meir.] The sages say: He may annul it.