Sotah Chapter 2, Mishnah 6. This mishnah continues the subject of extending the oath, whereby the sotah is made to swear about other relations she did not have as well. Our mishnah, however, teaches that there are situations that are not included in the extension of the oath, because they would not render the woman forbidden to her husband.
The text of the mishnah:
"Hakol shavin" - all agree, meaning the Sages and Rabbi Meir, who disagreed in the previous mishnah: here too they concede this ruling.
"She'eino masneh imah lo al kodem shetis'ares" - he does not stipulate with her and she is not made to swear about relations that took place before she became betrothed to her husband. And even if it was with that very same man, this does not render her forbidden to her husband, since it happened before their marriage.
"Velo al me'achar shetisgaresh" - nor about what happened after she was divorced from him. If he divorced her and afterward remarried her, the period in between is not included in the oath.
The mishnah goes on to spell out this case:
"Nistrah le'acher venitme'ah ve'achar kach hechezirah - lo hayah masneh imah" - she was secluded with another man after she was divorced, and it is possible that she had relations with him during that period, and afterward her husband took her back and remarried her. He does not stipulate with her and does not make her swear about that period, since this does not render her forbidden to him: a woman becomes forbidden to her husband only if she married another man in the interim, and not because of relations she had with another man.
"Zeh haklal":
"Kol shetiba'el velo haysah asurah lo - lo hayah masneh imah" - any act of relations that does not cause the woman to become forbidden to her husband is not something the husband stipulates with her about, and he does not make her swear about it, and there is no room here for extending the oath. He cannot make her swear about matters that do not bear directly on his ability to remain married to her.
In summary: in this mishnah we learned the limits of extending the oath in the case of a sotah: she is not made to swear about what preceded the betrothal, and she is not made to swear about what happened after the divorce, even if her husband took her back and remarried her. This is the rule: any act of relations that cannot render her forbidden to her husband is not included in the oath.