Parashas Vayeira · Passuk 20:16
And to Sarah he said, “Behold, I have given a thousand pieces of silver to your brother; behold, it is for you a covering of the eyes to all that are with you, and with all you are righted.”
Torah Temimah points to Bava Kamma 93a.
TT lemma: כסות עינים.
English: “A covering of the eyes.” Rabbi Yitzchak said: Never take a commoner’s blessing lightly, for Avimelech cursed Sarah and it was fulfilled in her offspring. As it is said, “Behold, it is for you a covering of the eyes”—he said to her, “Since you covered from me that he is your husband and caused me this distress, may it be that you have children who are covered of eyes.” And it was fulfilled in her offspring, as it is said of Yitzchak, “And his eyes were dim so that he could not see.”
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. A polite formula is turned into an effective imprecation whose fulfillment is traced to a later narrative.
Do not take lightly the blessing or curse of an ordinary person; Avimelech’s “covering of the eyes” was fulfilled in the dimming of Yitzchak’s eyes.