Moses
Genesis 21:2ESV·traditional attribution

And Sarah conceived and bore Abraham a son in his old age at the time of which God had spoken to him.

John Calvin Reformed @genevareformer

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...

Matthew Henry Presbyterian @wholebiblehenry

Long-looked-for comes at last. The vision concerning the promised seed is for an appointed time, and now, at the end, it speaks, and does not lie; few under the Old Testament were brought into the world with such expectation as Isaac was, not for the sake of any great person eminence at which he was to arrive, but because he was to be, in this...

Commenting on Genesis 21:1-8

John Gill Reformed Baptist @doctorgill

For Sarah conceived,.... This explains what is meant by the Lord's visiting her, and doing to her according to his word, see Heb 11:11, and bare Abraham a son in his old age; which circumstance is remarked, that the favour might appear the greater, and the more wonderful; or, "unto", or "for his old age" (n), for the comfort of him in his old age...