In Parshat Lech Lecha, God changed Avram’s name to Avraham, promised him and his descendents the Land of Israel and commanded him to perform the mitzvah of Brit Milah (the covenant of circumcision). God then changed Sarai’s name to Sarah.
In Breisheet 17:16, we read about Sarah’s blessing which was promised to Avraham: “I will bless her and I will also give you a son through her. I will bless her, and she will become the mother of nations, kings of peoples will descend from her.”
At that point (sentence 17), “Avraham fell on his face and laughed. He said in his heart: ‘Can a hundred year old man have children? Shall Sarah, who is ninety years old give birth?’”
Avraham then tried to convince God that Yishmael could be the heir. God explained that Sarah will indeed have a son named Yitzchak, the covenant will be through him and Sarah will give birth in exactly a year. Avraham then followed God’s instructions and proceeded to circumcise all of the men in his household.
In Parshat Vayera, three men came to visit Avraham. One of the men told him (Breisheet 18:10) “I will return next year, and Sarah, your wife will have a son.” Sarah heard this from the door of her tent.
In sentence 12 we read: “Sarah laughed to herself saying, ‘Now that I am worn out, shall I have the pleasure, my master being an old man.’”
God asks Avraham why Sarah laughed. This seems like a really strange question considering that Avraham himself laughed when he was given the same news the first time and God was not surprised when Avraham laughed. In fact, God even said that the child should be named Yitzchak, laughter!
Ramban explains that Sarah probably was unaware that the “men” were not regular men, rather angels sent with a prophecy from God. Since Sarah was in her tent, she most likely didn’t even see them. Avraham was so busy circumcising himself and his household that he did not have time to reveal to Sarah what God had originally told him, “Indeed, your wife, Sarah will bear you a son” so how could Sarah possibly have known that the man was serious when he said that she would give birth the following year?
When God is asked Avraham why Sarah laughed, he was actually saying, “Why was Sarah surprised? How did you not reveal the prophecy to her?”
We learn from here that even if you are in a rush to do a mitzvah, as Avraham was, sometimes you have to take a step back and realize that there is some news that just can’t wait!