Sotah 18B

ושומרת יבם, וכנוסה. אמן שלא נטמאתי, ואם נטמאתי ⁠— יבואו בי.

nor as a widow waiting for my yavam to perform levirate marriage, since a woman at that stage is prohibited from having sex with any men, nor when married through levirate marriage to the yavam; amen that I did not become defiled, and if I did become defiled, may all these curses come upon me.

רבי מאיר אומר: אמן שלא נטמאתי, אמן שלא אטמא.

R' Meir says that “amen, amen” means: Amen that I did not become defiled in the past, amen that I will not become defiled in the future.

הכל שוין שאין מתנה עמה, לא על קודם שתתארס ולא על אחר שתתגרש.

All agree that he may stipulate with her through this oath neither with regard to what she did before becoming betrothed to him, nor with regard to what she will do after she becomes divorced from him.

נסתרה לאחד ונטמאת, ואחר כך החזירה ⁠— לא היה מתנה עמה.

Similarly, if a husband divorced his wife, and while divorced she secluded herself with another man and became defiled, and afterward her husband took her back and remarried her, and he then warned her about a specific man, and she secluded herself, and she is now about to drink the water of the sota, he cannot stipulate with her that she take an oath that she did not become defiled during the period in which she was divorced. This is because her husband would become forbidden to her only if she had married another man after being divorced, not if she merely committed an act of promiscuity.

זה הכלל: כל שתבעל ולא היתה אסורה לו ⁠— לא היה מתנה עמה.

This is the principle: In every case where if she would have sex with someone else she would not become forbidden to her husband due to this act, he may not stipulate with her that her oath include that act. The oath can include only cases in which she would be rendered forbidden to him.