Genesis 21:2

For Sarah conceived, and bare Abraham a son in his old age at the set time of which God had spoken to him. God's promise was literally fulfilled, for at just the time that He had named at His last visit the son of promise was born, a stranger in truth, for Abraham was still sojourning in the land of the Philistines. The birth of Isaac was an act of faith on the part of Sarah, who with all her human infirmities was a true child of the Lord, Hebrews 11:11.