Genesis 21:1-5
21:1 Now the LORD was gracious to Sarah as he had said [God faithful to His word in every detail], and the LORD did for Sarah [90 years old] what he had promised.

21:2 Sarah became pregnant and bore a son to Abraham in his old age [100 years old], at the very time [precisely on divine schedule] God had promised him.

21:3 Abraham gave the name Isaac [cf. 17:19; name means “laughter”] to the son Sarah bore him.

21:4 When his son Isaac was eight days old, Abraham circumcised him, as God commanded [cf. Gen. 17:10] him.

21:5 Abraham was a hundred years old when his son Isaac was born to him [cf. Gen. 17:21].