Sotah 26B

מהו דתימא: ״נטמאה״ ״נטמאה״ שני פעמים: אחד לבעל ואחד לבועל. היכא דקא מיתסרא בהא זנות, אבל הא הואיל ואסורה וקיימא, אימא לא, קא משמע לן.

The Talmud answers: It is necessary, lest you say that the sota ritual does not apply with regard to forbidden relatives, as the Torah states: “And she was defiled” (Numbers 5:13), “And she was defiled” (Numbers 5:14), two times. One verse teaches that she is defiled and forbidden to her husband, and one verse teaches that she is forbidden to her paramour. One might understand that the sota ritual applies only where she is forbidden to the paramour due to this licentious sex; however, with regard to that woman, who secluded herself with a forbidden relative, since the woman already stands prohibited to him due to the prohibition of incest, one might say that the sota ritual does not apply. The Mishnah therefore teaches us that one can issue a warning even with regard to forbidden relatives.

חוץ מן הקטן וכו׳. ״איש״ אמר רחמנא, ולא קטן ושאינו איש. למעוטי מאי? אילימא למעוטי שחוף, והאמר שמואל: שחוף מקנין על ידו, ופוסל בתרומה!

§ The Mishnah states: A husband can issue a warning to his wife with regard to all those with whom sex are forbidden, with the exception of a minor and of one who is not a man. The Talmud cites the source for this halakha: God states in the Torah: “And a man lay with her” (Numbers 5:13), indicating that one can warn his wife with regard to a man but not with regard to a minor. The Talmud asks: The phrase: And of one who is not a man, serves to exclude what? If we say that it serves to exclude a sick man who lacks the ability to complete sex [shaḥuf], but didn’t Shmuel say: One can issue a warning with regard to a shaḥuf, and if a shaḥuf has sex with the daughter of a priest, he disqualifies her from partaking of teruma.