Parashas Vayeira · Passuk 18:14
Is anything too wondrous for Hashem? At the appointed time I will return to you, at this season, and Sarah shall have a son.
Torah Temimah points to Rosh Hashanah 11a.
TT lemma: למועד אשוב אליך.
English: “At the appointed time I will return to you.” From here we learn that Yitzchak was born on Pesach, for He was standing [speaking] on Pesach and said to her, “At the appointed time I will return to you, at this season, and Sarah shall have a son.”
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The timing language fixes the birth date relative to the announcement.
Yitzchak was born on Pesach; the promise was made on Pesach and the birth occurred “at this season” one year later.