Nedarim, Chapter 10, Mishnah 7. This Mishnah deals with a husband's ability to annul his wife's vows in advance, or to uphold them in advance - before she has even taken them.
Upholding vows in advance:
The Mishnah opens: "HaOmer l'ishto: kol hanedarim shetidori mikan ad she'avo mimakom peloni - harei hen kayamin" - one who says to his wife: all the vows you take from now until I return from such and such a place are hereby upheld. A husband setting out on a journey wishes to uphold in advance every vow his wife will take from that moment until he returns, giving them binding force. On this the Mishnah rules: "lo amar kelum" - his words have no substance, and the upholding does not take effect on vows that have not yet been taken.
Some explain the reason: it is quite possible that when he actually hears the vow he will not want it to stand, so his upholding turns out to have been made in error, and for that reason it is not valid.
Annulling vows in advance - a dispute:
And what is the law when he said "harei hen mufarin" - that all the vows his wife takes from now until he returns should be annulled and never take effect at all?
"Rabbi Eliezer omer: mufar" - Rabbi Eliezer says the annulment is valid in advance, even though the vows have not yet been taken.
"VaChachamim omrim: eino mufar" - and the Sages say it is not annulled, for he has no power to annul vows that have not yet come into existence.
Rabbi Eliezer's argument:
Rabbi Eliezer said: "Im hefer nedarim sheba'u likhlal issur, lo yafer nedarim shelo ba'u likhlal issur?" - if annulment is effective even for vows that have already taken effect and reached the status of a prohibition, then all the more so (kal vachomer) it should be effective for vows that have not yet taken effect, since their force is not as strong as that of the former.
The Sages' reply:
They said to him: "Harei hu omer: ishah yekimenu v'ishah yefirenu" - the verse joins upholding and annulment together, and from here we learn that their law is the same:
"Es sheba likhlal hakem - ba likhlal hafer" - a vow that has reached a state in which it can be upheld, meaning that she has already taken it, can also be annulled.
"Lo va likhlal hakem - lo va likhlal hafer" - a vow that is not yet subject to upholding cannot be annulled either.
In summary: in this Mishnah we learned that a husband cannot uphold his wife's vows in advance, and according to the Sages he cannot annul them in advance either. Rabbi Eliezer reasoned by kal vachomer that the power of annulment extends even to vows that have not yet taken effect, while the Sages replied from the verse "ishah yekimenu v'ishah yefirenu" that annulment and upholding are equivalent to one another: whatever is not subject to upholding is not subject to annulment.