Ambrose of Milan

But if the words of Abraham are not enough to correct, consider the word of God, who condemns such a mode of transmitting inheritance. "This man shall not be your heir," he says, "but the other who will come out from you, he will be your heir." Who is this other of whom he speaks? In fact Hagar too bore a son, Ishmael, but he is not speaking of him. Instead, he is speaking of holy Isaac. …

On Abraham ↗

Bede

This one shall not be your heir; but one who will come from your own body shall be your heir. This is proved by reason to have been said about Isaac and his offspring; of whom it was later heard: In Isaac shall your seed be called. For the sons of the concubines, although they were the seed of Abraham, could not be partakers of his inheritance; and since to this inheritance beloved by God, in which the divine protection and great reward would exist, only heavenly souls pertain, it is fittingly added:

Commentary on Genesis (Hexaemeron) ↗