Parashas Toldos · Passuk 26:8
When some time had passed, Avimelech king of the Philistines, looking out of the window, saw Yitzchak fondling Rivkah his wife.
Rashi on Bereishis 26:8 / Bereshit Rabbah (classical source on this passuk; not listed as a numbered Torah Temimah unit here).
Lemma / focus: כי ארכו / וישקף.
English: He said: From now I need not worry, since they have not forced her until now—and he was not careful to remain guarded. Avimelech looked and saw him engaged in marital relations.
Derash on the verse (Rashi / midrash) — not a formal named middah of the 13: the length of days explains the lapse of caution, and the looking is specified as seeing intimacy.
When days lengthened, Yitzchak was less guarded; Avimelech looked through the window and saw that Rivkah was his wife.