Genesis 21:5

“And Abraham was an hundred years old when son Isaac was born unto him.] So that this was years after his departure from Haran, and coming into the land of Canaan, for then he was seventy five years of age, (<011204>Genesis 12:4); and this exactly agrees with the account of Demetrius, as related by Polyhistor, an Heathen writer f1211, who makes Isaac to be born just twenty five years from Abraham’s coming into the land of Canaan, and who must be now an hundred years old, being ninety nine at the time the Lord appeared unto him, and promised him a son at the set time the next year, (<011701>Genesis 17:1,21). This is observed, both to show the wonderful favour to Abraham, and the faithfulness…”

“21:5 Isaac was born twenty-five years after the promise was first given (cp. 12:4).”