John Gill

Of whom it is said,.... That is, of Isaac, Abraham's own son, whom he offered up; or rather, "to whom it was said", as the Vulgate Latin and Syriac versions render it; that is, to Abraham, for to him was this said, Gen 21:12 that in Isaac shall thy seed be called; that numerous natural seed of his, which should inherit the land of Canaan; and his special famous seed, the Messiah, to whom the promises were made; See Gill on Rom 9:7.