Parashas Vayeira · Passuk 21:12
And God said to Avraham, “Let it not be bad in your eyes concerning the lad and concerning your maidservant; whatever Sarah says to you, listen to her voice, for in Yitzchak shall seed be called to you.”
Torah Temimah points to Sanhedrin 59b.
TT lemma: כי ביצחק.
English: “For in Yitzchak…” Why were the children of Yishmael not obligated in circumcision…? Because it is written “for in Yitzchak shall seed be called to you.” If so, should the children of Esav be obligated? “In Yitzchak”—but not all of Yitzchak.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The covenantal language is narrowed by the same restrictive reading used for vows.
Neither the descendants of Yishmael nor the descendants of Esav are bound by the perpetual covenant of circumcision; only the line designated “in Yitzchak” carries that obligation.