Parashas Toldos · Passuk 25:20
Yitzchak was forty years old when he took Rivkah, daughter of Betuel the Aramean of Paddan-aram, sister of Lavan the Aramean, as his wife.
Rashi on Bereishis 25:20 (classical source on this passuk; not listed as a numbered Torah Temimah unit here).
Lemma / focus: בן ארבעים שנה.
English: When Avraham returned from Mount Moriah he was told that Rivkah had been born, and Yitzchak was then thirty-seven… He waited ten years until she was fit for marriage and married her, and then waited another ten years until she was thirteen… thus he was forty.
Derash on the verse (Rashi / midrashic chronology) — not a formal named middah of the 13: the stated age is read against the narrative chronology from Mount Moriah.
Yitzchak was forty when he married Rivkah; the years from the Akeidah and her birth until the marriage account for that age.