John Gill Commentary Genesis 21:1

"And Jehovah visited Sarah as he had said, and Jehovah did unto Sarah as he had spoken." — Genesis 21:1 (ASV)

And the Lord visited Sarah as he had said To Abraham, (Genesis 17:16Genesis 17:19Genesis 17:21) (Genesis 18:10Genesis 18:14); in a way of mercy and kindness, by fulfilling his promise, giving strength to conceive and bear a child; see (1 Samuel 2:21).

and the Lord did unto Sarah as he had spoken; which intends the same thing in different words; and the repetition is made to cause attention to God's fulfilment of his promise, who is always faithful to his word, even in things very difficult and seemingly impossible, as in the present case: hence the Targums of Jonathan and Jerusalem paraphrase it, God did a wonder or wonders for Sarah in causing her to conceive when she was so old, and in such circumstances as she was.