Genesis 21:1-2:

And the LORD visited Sarah as he had said, and the LORD did unto Sarah as he had spoken.
For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him.

Notes on Genesis 21:1-2:

If the Lord visited Sarah after Sarah believed God’s word to her, you can have the Lord visit you so that you can conceive after you believe God’s Word.

See also:

Condition: Infertility