Maseches Sotah, chapter 3, Mishnah 5. This Mishnah presents the opinion of Rabbi Shimon, who disagrees with what was stated in the previous Mishnah - that at times a woman's merit prevents her from dying after drinking the bitter waters.
The opinion of Rabbi Shimon:
"Ein zechus tolah lamayim" - merit does not cause her to remain alive after she has drunk the bitter waters.
And his reasoning: "V'im tomar hazechus tolah lamayim" - if you wish to say that merit saves her from the bitter waters, two problems result:
"Madheh atah es hamayim bifnei kol hanashim hashosos" - you weaken the effectiveness of the waters in the eyes of all the women who drink them.
"U'motzi atah shem ra al hat'horos sheshasu" - beyond diminishing the effectiveness of the waters, you cast a bad name upon the women who were entirely innocent and drank.
For people will end up saying about them: "Temei'os hen, ela shetalsah lahen zechus" - they really did commit adultery, and how did they survive? Because merit stood by them and saved them. For this reason Rabbi Shimon holds that this is not the case: no merit stands for her at the time of drinking, and if she is guilty, she dies.
The opinion of Rabbi - a third approach:
Rabbi says: "Hazechus tolah lamayim" - merit does indeed prevent her from dying. At the same time, however, she is not blessed in the way that an innocent woman is blessed. The Torah teaches that an innocent woman who drinks the sotah waters is blessed: if she was barren, she begins to bear children, and if she already bore children, she now gives birth more easily, and her children are of finer quality.
This woman, by contrast, who is guilty but has merit, "V'einah yoledes v'einah mashbachas" - she does not merit to give birth, and her condition does not become good and improved, "ela misnavenah v'holeches" - on the contrary, she begins to waste away, "u'lesof b'osah misah meisah" - ultimately, after a longer and more drawn out period, she dies the very same death she would have died were it not for her merit: the illness appears in her, her body swells, and so on.
Accordingly, according to Rabbi, none of the problems raised by Rabbi Shimon arise: on the one hand there is merit that prevents immediate death, and on the other hand one can clearly distinguish between a woman for whom merit merely suspended the punishment and a woman who was truly innocent.