ויאמר שוב אשוב אליך כעת חיה והנה-בן לשרה אשתך ושרה שמעת פתח האהל וגו’ ותצחק שרה בקרבה לאמר אחרי בלתי היתה-לי עדנה ואדני זקן
[The angel] said: “I shall surely return to you, at this time next year, and behold! Sarah your wife will have a son.” Sarah overheard this by the entrance to the tent…Sarah laughed inwardly, saying: “After I have dried up, I would have youthfulness? And my husband is old!”
The commentaries are bothered by Sarah’s lack of belief that she could bear a son. After all, it was an Angel of Hashem telling her that she would have a son, despite her old age. Now, some opinions hold that she and Avraham didn’t realize that these were Angels speaking to them. Or, even if Avraham knew, perhaps she didn’t see who was speaking; she just overheard some stranger saying something that sounded crazy. However, we still have a problem. Hashem Himself told Avraham in last week’s parsha that he and Sarah would have a baby boy. Why didn’t Sarah believe this supposed stranger then, who said the same thing?
We have no choice but to say that Avraham didn’t tell Sarah about this prophecy. Why would that be? We can only speculate. The Ramban suggests two possibilities. Either Avraham felt that since Hashem didn’t give him permission to share the news, this prophecy was meant to be a personal one. Perhaps Hashem will reveal to Sarah the news at the right time.
Alternatively, we see from the chronology that Avraham didn’t really have a chance to tell Sarah. Immediately upon hearing the news of a son, Avraham was commanded in bris milah. He right away got to work and circumcised himself and his whole household. This was obviously not a simple procedure, and he needed to recover. Our parsha picks up three days after his bris milah. Our Sages say that the third day is the most painful. It’s not surprising then that Avraham didn’t get around to telling Sarah. He was probably planning to tell her once his wound healed.
Rav Yosef Dov Soloveitchik, known as the Beis HaLevi, shares an interesting insight into the greatness of Avraham. He notes that despite being so busy with the bris milah and the subsequent recovery process, this didn’t deter him hosting the three Angels, in the guise of nomadic guests. He didn’t even have a chance or the strength to tell Sarah that after all these years, they were finally going to have a child. Nevertheless, when he saw seemingly starving desert wanderers, he jumped at the chance to take care of their physical needs. We see just how precious the mitzvah of hosting guests was for Avraham Avinu.
 Based on the Brisker Rav’s Chiddushei HaGriz to Genesis 18:15
 Genesis 18:11,13
 Rashi et. al.
 Ramban to v. 15
 Rashi to v. 1