John Calvin on Genesis 21:2

2. She bare Abraham a son . This is said according to the accustomed manner of speaking; because the woman is neither the head of a family, nor brings forth properly for herself, but for her husband. What follows, however, is more worthy of notice, In his old age, at the set time, which God had predicted: for the old age of Abraham does, not a little, illustrate the glory of the miracle. And now Moses, for the third time, recalls us to the word of God, that the constancy of his truth may always be present to our minds. And though the time had been predicted, alike to Abraham and to his wife, yet this honor is expressly attributed to the holy man; because the promise had been especially given on his account. Both, however, are distinctly mentioned in the context.