Talmud for Nazir 5:4
מִי שֶׁנָּדַר בְּנָזִיר וְהָלַךְ לְהָבִיא אֶת בְּהֶמְתּוֹ וּמְצָאָהּ שֶׁנִּגְנְבָה, אִם עַד שֶׁלֹּא נִגְנְבָה בְהֶמְתּוֹ נָזַר, הֲרֵי זֶה נָזִיר. וְאִם מִשֶּׁנִּגְנְבָה בְהֶמְתּוֹ נָזַר, אֵינוֹ נָזִיר. וְזוֹ טָעוּת טָעָה נַחוּם הַמָּדִי כְּשֶׁעָלוּ נְזִירִים מִן הַגּוֹלָה וּמָצְאוּ בֵית הַמִּקְדָּשׁ חָרֵב, אָמַר לָהֶם נַחוּם הַמָּדִי, אִלּוּ הֱיִיתֶם יוֹדְעִים שֶׁבֵּית הַמִּקְדָּשׁ חָרֵב הֱיִיתֶם נוֹזְרִים. אָמְרוּ לוֹ לֹא, וְהִתִּירָן נַחוּם הַמָּדִי. וּכְשֶׁבָּא הַדָּבָר אֵצֶל חֲכָמִים, אָמְרוּ לוֹ, כֹּל שֶׁנָּזַר עַד שֶׁלֹּא חָרַב בֵּית הַמִּקְדָּשׁ, נָזִיר. וּמִשֶּׁחָרַב בֵּית הַמִּקְדָּשׁ, אֵינוֹ נָזִיר:
If one vowed to be a Nazirite and went to bring his beast and found that it had been stolen, [When he made the vow, he (assumed that) he had beasts, and it was on the basis of this assumption that he vowed, intending his Nazirite offerings to come from these beasts. And he went and found that they had been stolen, so that he regretted having vowed to be a Nazirite] — If he had vowed before his beast were stolen, he is a Nazirite [And the sage cannot absolve him of his vow with this "opening," for it is "nolad" (something "born" after the vow), and "nolad" cannot be used as an opening.], and if he had vowed after his beast were stolen, he is not a Nazirite. [If he said: "Had I known that it had been stolen, I would not have vowed, this is an opening, and the sage can absolve him of the vow.] And Nachum Hamadi fell into this error when the Nazirites came up from the exile and found that the Temple had been destroyed. [They had vowed before the Temple had been destroyed, and he absolved them, and the sages said to him that this was nolad, which may not be used as an opening. The halachah is in accordance with the sages.] Nachum Hamadi said to them: "Had you known that the Temple had been destroyed, would you have vowed?" They answered in the negative, and he absolved them. And when the sages were apprised of it, they said to him: Everyone who vowed Naziritism before the Temple had been destroyed is a Nazirite. (Everyone who vowed Naziritism) after the Temple had been destroyed is not a Nazirite.