Romans 9:7

Neither are all the bodily descendants of Abraham also his spiritual descendants. It was expressly stated from the first that the promise was confined to a particular branch of his posterity. The posterity of Abraham, strictly so called, was to be that derived through Isaac. This is very nearly the sense of the original, “In Isaac shall thy seed be called,” i.e., in “Isaac shalt thou have posterity, which shall be called thy posterity” — “true and legitimate descendants,” thus excluding the seed of Hagar.