The LORD was gracious to Sarah, just as he had said he would be. He did for Sarah what he had promised to do.
References for Genesis 21:1
Sarah became pregnant. She had a son by Abraham when he was old. He was born at the exact time God had promised him.
Abraham gave the name Isaac to the son Sarah had by him.
When his son Isaac was eight days old, Abraham circumcised him. He did it exactly as God had commanded him.
Abraham was 100 years old when his son Isaac was born to him.
Sarah said, "God has given laughter to me. Everyone who hears about this will laugh with me."
She continued, "Who would have said to Abraham that Sarah would nurse children? But I've had a son by him when he is old."