Hebrews 11:18

Of whom it was said, That in Isaac shall thy seed be called:

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, Genesis 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 "Romans 9:7".