This reading is from the Torah portion read on the first day of Rosh Hashanah, which falls on September 27 this year.
The Lord took note of Sarah as He had promised, and the Lord did for Sarah as He had spoken. Sarah conceived and bore a son to Abraham in his old age, at the set time of which God had spoken. Abraham gave his newborn son, whom Sarah had borne him, the name of Isaac. And when his son Isaac was eight days old, Abraham circumcised him, as God had commanded him. Now Abraham was a hundred years old when his son Isaac was born to him. Sarah said, "God has brought me laughter; everyone who hears will laugh with me."
And she added, "Who would have said to Abraham
That Sarah would suckle children!
Yet I have borne a son in his old age."
The child grew up and was weaned, and Abraham held a great feast on the day that Isaac was weaned.
Sarah saw the son whom Hagar the Egyptian had borne to Abraham playing. She said to Abraham, "Cast out that slave-woman and her son, for the son of that slave shall not share in the inheritance with my son Isaac." The matter distressed Abraham greatly, for it concerned a son of his. But God said to Abraham, "Do not be distressed over the boy or your slave; whatever Sarah tells you, do as she says, for it is through Isaac that offspring shall be continued for you. As for the son of the slave-woman, I will make a nation of him, too, for he is your seed."