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 Nedarim 31a.
TT lemma: כי ביצחק.
English: “For in Yitzchak…” One who says, “Konam that I will not benefit from the seed of Avraham” is forbidden to benefit from Israelites and permitted to benefit from non-Jews. But is there not Yishmael? “For in Yitzchak shall seed be called to you.” And is there not Esav? “In Yitzchak”—but not all of Yitzchak.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The phrase limits the legal category of Avraham’s seed.
“Seed of Avraham” for the purpose of a vow excludes Yishmael and Esav; only the line of Yitzchak that is “called” is included, and not every descendant of Yitzchak.