Sanhedrin 53B

איסור מצוה ⁠– שניות מדברי סופרים. ואמאי קרו להו איסור מצוה? שמצוה לשמוע דברי חכמים.

The Mishnah explains: A prohibition resulting from a mitzva is referring to secondary forbidden relatives, e.g., one’s son’s daughter-in-law, who are forbidden by rabbinic law. The rabbis instituted this prohibition in order to distance people from incest. The Talmud explains: And why do the rabbis call it a prohibition resulting from a mitzva? Because it is a mitzva to listen to the words of the rabbis.

איסור קדושה ⁠– אלמנה לכהן גדול, גרושה וחלוצה לכהן הדיוט. ואמאי קרו להו איסור קדושה? דכתיב: ״קדשים יהיו לאלהיהם״.

The Mishnah continues: A prohibition stemming from sanctity is referring to the marriage of a widow to a High Priest, or to the marriage of a divorcée or of a woman who has performed ḥalitza [ḥalutza], to an ordinary priest. The Talmud explains: And why do the rabbis call it a prohibition stemming from sanctity? As it is written with regard to the priests: “They should be sanctified to their God” (Leviticus 21:6).

ותניא עלה: רבי יהודה מחליף.

And it is taught in a baraita with regard to this Mishnah: R' Yehuda switches the terms; he refers to the marriage of a divorcée or of a ḥalutza to a priest as a prohibition resulting from a mitzva, and to the marriage of secondary relatives as a prohibition stemming from sanctity.

חלופי הוא דמחליף, הא חליצה בעיא. ואי סלקא דעתך רבי יהודה כרבי עקיבא סבירא ליה, מכדי חייבי לאוין לרבי עקיבא כחייבי כריתות דמי, וחייבי כריתות לאו בני חליצה ויבום נינהו!

The Talmud infers: He switches the terms but does not dispute the halakha; but even R' Yehuda agrees that if a priest marries a ḥalutza and then dies, she requires ḥalitza. And if it enters your mind that R' Yehuda holds in accordance with the opinion of R' Akiva, this poses a difficulty; after all, those liable for violating prohibitions, according to the opinion of R' Akiva, are considered like those liable to receive karet, and those liable to receive karet are not capable of performing ḥalitza and levirate marriage, as the mitzva of levirate marriage does not apply to such relationships.