Midrash for Numbers 30:9

"This is the thing": We are hereby apprised that just as the prophets prophesied by "Thus said the L-rd," so did Moses, viz. (Shemot 11:4), and, in addition, "This is the thing, etc." Variantly: This is the "word" (for the absolution of vows.) A husband "annuls" ("mefer" [his wife's vows, viz. Bamidbar 30:9]), but a sage does not annul. A sage "permits" ("matir" [viz. Bamidbar 30:3]) but a husband does not permit. For would it not follow otherwise, viz.: If he who does not annul, permits, then he who does annul, how much more so should he permit. And if he who does not permit, annuls, then he who does permit, how much more so should he annul! It is, therefore, written "This is the 'word' that the L-rd has commanded": The husband "annuls," and a sage does not annul. A sage "permits," and the husband does not permit.

(Bamidbar 30:9) "And if on the day that her husband hear, he constrain her and annul her vow": We learn vis-à-vis the husband that "constraint" is annulment and vis-à-vis the husband that silence on the day of his hearing is equated with (silence on) the day of the vow.