Yevamot 92B

אמר רב: מנין שאין קדושין תופסין ביבמה, שנאמר: ״לא תהיה אשת המת החוצה לאיש זר״⁠ — לא תהא בה הויה לזר.

that Rav said: From where is it derived that betrothal by another man does not take effect with a yevama? As it is stated: “The wife of the dead man should not be married outside of the family to one not of his kin” (Deuteronomy 25:5), which indicates: She should not have the possibility of becoming married to one not of his kin, i.e., his betrothal is of no account.

ושמואל אמר: בעניותינו, צריכה גט. מספקא ליה לשמואל האי ״לא תהיה אשת המת״, אי ללאו הוא דאתא, אי דלא תפסי בה קדושין הוא דאתא.

And Shmuel said: In our poverty of knowledge, as we do not fully understand the verse, she requires a bill of divorce. The Talmud explains: Shmuel was uncertain with regard to this verse: “The wife of the dead man should not be married outside,” whether it comes for a prohibition, i.e., the woman is prohibited from marrying another man but the betrothal of that other man is effective, or whether it comes to teach that betrothal by any other man does not take effect with her.

אמר ליה רב מרי בר רחל לרב אשי, הכי אמר אמימר: הלכה כוותיה דשמואל. אמר רב אשי: השתא דאמר אמימר הלכתא כוותיה דשמואל ⁠— אם היה יבמה כהן, חולץ לה, ושריא ליה.

Rav Mari bar Raḥel said to Rav Ashi that Ameimar said as follows: The halakha is in accordance with the opinion of Shmuel. Rav Ashi said: Now that Ameimar said that the halakha is in accordance with the opinion of Shmuel, who maintains that one who betroths a yevama before she has performed ḥalitza must give her a bill of divorce, if her yavam was a priest, he performs ḥalitza with her, as she is forbidden to him after the man who betrothed her gave her a bill of divorce, and she is thereby permitted to the man to whom she was betrothed.