Nazir 32B

ואם משנגנבה בהמתו נזר ⁠— אינו נזיר. וזו טעות טעה נחום המדי: כשעלו נזירים מן הגולה ומצאו בית המקדש חרב, אמר להם נחום המדי: אילו הייתם יודעין שבית המקדש חרב, הייתם נוזרים? אמרו לו: לא. והתירן נחום המדי.

But if he took a vow of naziriteship after his animal was stolen, he is not a nazirite, as it is retroactively established that his vow was taken in error from the outset, as he relied on an animal he did not possess. And this was the error that Naḥum the Mede erred when he failed to distinguish between an event that occurred before the vow was taken and an event that occurred afterward. The incident in question was as follows: When nazirites were ascending from the exile to sacrifice their offerings, and they found the Temple destroyed, Naḥum the Mede said to them: If you had known that the Temple would be destroyed, would you have taken a vow of naziriteship? They said to him: Certainly not, as there is no remedy for a naziriteship in this case. And Naḥum the Mede dissolved the vow for them.

וכשבא הדבר אצל חכמים אמרו: כל שנזר עד שלא חרב בית המקדש ⁠— נזיר. ומשחרב בית המקדש ⁠— אינו נזיר.

And when the matter came before the Rabbis, they said: His ruling is incorrect. Rather, whoever took a vow of naziriteship before the Temple was destroyed, like these nazirites from the exile, he is a nazirite, as he committed no error at the time of his vow, and one cannot dissolve vows based a new situation. However, one who stated his vow after the Temple was destroyed is not a nazirite, as he vowed based on an erroneous assumption.

גמ׳ אמר רבה: שטפוהו רבנן לרבי אליעזר ואוקמיה בשיטתייהו. דתנן: פותחין בנולד, דברי רבי אליעזר, וחכמים אוסרין.

Talmud: In relation to the Mishnah’s statement with regard to the dissolution of a vow of naziriteship due to a new situation, the Talmud cites a statement that Rabba said: The Rabbis overwhelmed R' Eliezer until he retracted his ruling and established the halakha in accordance with their opinion. To what does this refer? As we learned in a Mishnah in Nedarim (64a): They may broach dissolution by asking about a new situation, i.e., a halakhic authority can dissolve a vow due to a new situation that the one who took the vow did not anticipate at the time he took his vow. This is the statement of R' Eliezer; but the Rabbis prohibit this. Since R' Eliezer does not disagree in the case of naziriteship in this Mishnah, he must have accepted the opinion of the Rabbis.