John Gill Commentary Hebrews 11:18

"even he to whom it was said, In Isaac shall thy seed be called:" — Hebrews 11:18 (ASV)

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 your 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).