EXTRA · Parashas Vayeira · Passuk 21:10
Supplementary classical material; Torah Temimah lists no citation under this passuk.
She said to Avraham, “Drive out this maidservant and her son, for the son of this maidservant shall not inherit with my son, with Yitzchak.”
Rashi on Genesis 21:10 (citing Bereishis Rabbah).
Sefaria — Rashi on Genesis 21:10
English: “With my son, with Yitzchak.” Since he is my son — even if he were not as worthy as Yitzchak — or if he were as worthy as Yitzchak even if he were not my son — he would not be fit to inherit with him; all the more so “with my son, with Yitzchak,” both qualities being present.
Derash on the verse (Rashi / Midrash) — not a formal named middah of the 13. The doubled language is treated as intensifying exclusion by a fortiori reasoning.
Inheritance of the covenant line is guarded by both natural filiation and moral fitness; Sarah’s insistence protects the unique status of Yitzchak.