Yevamot 37A

באורתא אמר רבא הכי, ובצפרא הדר ביה. אמר ליה: שריתוה? יהא רעוא דתש(ת)רו אף תרבא.

In the evening Rava stated this ruling in accordance with the way you cited him, but in the morning he retracted his opinion and ruled as I cited him. Rav Mesharshiyya said to him: Do you really permit her to marry without ḥalitza? He added sarcastically: May it be God’s will that you will even permit forbidden fats [tarba] as well. In Rav Mesharshiyya’s opinion, the prohibition against the widow remarrying without ḥalitza was as obvious as the prohibition of forbidden fats.

הכא גבי מעוברת חבירו ומינקת חבירו הנשואה לכהן, מאי? מי עבוד רבנן תקנתא לכהן, או לא.

After citing this dispute, Rav Ashi said to Rav Hoshaya, son of Rav Idi: According to Rava’s citation of Rav, when a woman is married to a priest, the rabbis were more lenient in order to allow the couple to remain married. Here, with regard to a woman who is pregnant with the child of another man or a woman who is nursing the child of another man, who is married to a priest, what is the halakha? Did the rabbis enact an ordinance for the benefit of a priest and say that it is sufficient for him to separate from his wife and he does not need to divorce her, or not?

אמר ליה: הכי השתא? בשלמא התם, כיון דאיכא רבנן דפליגי עליה דרבן שמעון בן גמליאל, דאמרי: אף על גב דלא שהא ולד מעליא הוי, גבי אשת כהן כיון דלא אפשר ⁠— עבדינן כרבנן,

He said to him: How can these cases be compared? Granted, there, in the case where the offspring died during its first 30 days of life, since there are the Rabbis who disagree with Rabban Shimon ben Gamliel concerning it, as they say that although the offspring did not survive for 30 days it is nevertheless considered a full-fledged, i.e., viable, offspring, therefore, with regard to the wife of a priest, since it is not possible for her to perform ḥalitza and remain permitted to her husband, we will be lenient and act in accordance with the opinion of the Rabbis.