Nedarim 151:1-152:99:1

אימא סיפא אמרו לו לר"א אם מטבילין כלי טמא ליטהר יטבילו כלי לכשיטמא ליטהר שמע מינה חיילין

Then consider the second clause: They [the Rabbis] said to R. Eliezer: If an unclean utensil is immersed in order to purify it, shall a clean utensil be immersed, so that on [subsequently] becoming defiled it shall [simultaneously] become clean!<span class="x" onmousemove="('comment',' Surely not. ');">¹</span> This proves that they do take effect.<span class="x" onmousemove="('comment',' Since they compare it to the prior immersion of a utensil to render it clean after it has become defiled. ');">²</span>

אמרי רבנן לא קיימי להון בטעמיה דר"א והכי קאמרי ליה מאי סבירא לך אי סבירא לך דחיילין ובטלין תהוי כלי תיובתך אי לא סבירא לך דחיילין תהוי מקוה תיובתך

— I will tell you: The Rabbis were not clear as to R. Eliezer's standpoint. Hence they said thus to him: What is your opinion? If you maintain that they [the vows] take effect, but are annulled, you are refuted by [the analogy of] a utensil; whilst if you do not hold that they take effect, the mikweh is your refutation. Come and hear: R. Eliezer said to them: If defiled seeds are rendered clean by being sown in the soil, how much more so if [already] sown and rooted [in the soil]!<span class="x" onmousemove="('comment',' That they certainly cannot be defiled. Thus also vows: if a vow can be annulled when already in force, surely the annulment can operate to prevent it from coming into force! ');">³</span>

ת"ש אמר להם ר"א ומה זרעים טמאים כיון שזרען בקרקע טהורין זרועין ועומדים לא כל שכן ש"מ לא חיילין

This proves that they do not take effect at all. Now, do not the Rabbis admit the validity of [such] an ad majus conclusion? Surely it was taught: I might think that a man can sell his daughter when a na'arah:<span class="x" onmousemove="('comment',' The reference is to Ex. XXI, 7. ');">⁴</span>

ורבנן לא דרשי ק"ו והא תניא יכול ימכור אדם את בתו כשהיא נערה אמרת ק"ו מכורה כבר יוצאה אינה מכורה אינו דין שלא תימכר

— But you can argue a minori: if she who was already sold goes free,<span class="x" onmousemove="('comment',' On attaining the na'arah stage. ');">⁵</span> is it not logical that if not sold yet, she cannot be sold [now]!<span class="x" onmousemove="('comment',' V. Kid. 4a. This reasoning is exactly analogous to R. Eliezer's. The Talmud interposes that no verse is required. ');">⁶</span>