Genesis 21:2

For Sarah conceived and bore Abraham a son in his old age, at the set time of which God had spoken to him.

Surrounding context

21:1 And the LORD visited Sarah as He had said, and the LORD did for Sarah as He had spoken.

21:2 For Sarah conceived and bore Abraham a son in his old age, at the set time of which God had spoken to him.

21:3 And Abraham called the name of his son who was born to him--whom Sarah bore to him--Isaac.