Midrasch zu Jeschijahu 65:15

(Bamidbar 5:27) "and her belly will swell and her thigh will fall": This tells me only of her belly and her thigh. Whence do I derive (the same for) the rest of her limbs? From "then the blighting waters will enter into her." — Let only this be stated, then. Why need it be added "and her belly will swell and her thigh will fall"? From that limb whence the sin began, from that limb will the punishment begin! Similarly, (Bereshit 7:23) "And He blotted out every being upon the face of the ground — from man until beast." He who began the sin, from him will the punishment begin! Similarly, (Ibid. 19:11) "And the men at the entrance of the house they smote with blindness, from small to great." They who began the sin, from them the punishment began. Similarly, (Shemot 14:4) "and I will be honored through (the downfall of) Pharaoh and all of his host." Pharaoh began the sin — from him the punishment began. Similarly, (Devarim 15:16) "Smite shall you smite the inhabitants of that city by the sword. Lay it waste and all that is in it, etc." Whence the sin began, the punishment began. Here, too, "and her belly will swell and her thigh will fall." From that limb whence the sin began, from it the punishment began. Now does this not follow a fortiori. If re the attribute of punishment, the weaker attribute — the limb whence the sin began, from it the punishment begins, how much more so re the attribute of benefaction, the stronger attribute, (the limb whence the good began, from it the reward begins!) "and the woman will be a curse in the midst of her people": They will curse through her — "May it happen to you as it happened to her!" "for an oath" (see verse 21): They will swear by her — ("I swear that if, etc.,) may it happen to me as it happen to her!" And thus is it written (Isaiah 65:15): "And you (the wicked) will leave your name as an oath for My chosen ones" — whence we learn that the wicked are an oath for the righteous. And whence do we derive that the righteous are a blessing for the wicked? (Jeremiah 4:2) "And in it (Israel) will nations bless themselves, and in it will they be praised," and (Bereshit 12:3) "And there will bless themselves in you (Avram) all the families of the earth." And it is written (Ibid. 48:20) "And he blessed them on that day, saying: In you (Ephraim and Menasheh) will Israel bless, etc."