RASHI on BERAISHIS 18:9 (end of ויאמרו אליו) quotes Gemara in Baba Metzia 87a
- רבי יוסי ברבי חנינא אמר כדי לשגר לה כוס של ברכה - Rabbi Yosei, son of Rabbi Ḥanina, says: They [the Angels] inquired about her [Sarah] in order to send her the cup of [wine of the] blessing.
The Sifsei Chachamim there (20-כ) says:
ואברהם לא היה רוצה לשלוח לה לפי שפרסה נדה ולא תטמא המשקין עם הכוס, והוא לא רצה לגלות שפירסה נדה לכן לא רצה לשפוך לכוס אחרולשלוח לה. - Avraham Did not want to send her [the the cup of wine of the blessing, by himself], because she had begun to menstruate, and so that she should not cause the wine and the cup to become tamei impure. And he did not want to publicize that she had begun to menstruate, therefore he did not want to pour it [the wine] into another cup (saving his own cup from becoming tamei), and send it to her:
Question: Even if the suggestion of the Sifsei Chachamim of pouring the wine into another cup would have solved the issue concerning the wine and cup becoming tamei, Avraham should also have been concerned about the Rabbinical prohibition of a husband sending his wife wine while she is a nidah? And this could have been the reason why he did not use the idea suggested by the Sifsei Chachamim to pour it [the wine] into another cup and send it to Sarah, because pouring into another cup especially for her is prohibited when she is a nidah.
See My question here asking whether Avraham kept harchakos of niddah.