Midrash su Numeri 30:16

(Bamidbar 30:16) "And if he annul them after his hearing": after his confirmation of the vow. You say this, but perhaps it is "after his hearing," literally? (This cannot be, for) "for he was silent to her on the day of his hearing" already speaks of hearing. How, then, am I to understand "after his hearing"? As after his confirmation of the vow. "And if he annul them after his hearing," i.e., after his confirmation (and she breaks her vow), "then he shall bear her sin." (i.e., he takes her place for (punishment of) the sin. Now does this not follow a fortiori, viz. If in respect to G-d's measure of punishment, which (relative to that of reward) is small, one who causes his neighbor to go astray takes his place for punishment, then, in respect to His measure of good, which is (relatively) large, how much more so (is one rewarded for being instrumental in his neighbor's mitzvah)!