Reference for Megillah 55:16

ר"א אמר עיניו כהות שנאמר (בראשית כז, א) ויהי כי זקן יצחק ותכהין עיניו מראות משום דאסתכל בעשו הרשע

His eyes become dim, as it says, “And it came to pass that when Yitzchak grew old that his eyes became dim, so that he could not see” (Genesis 27:1). Because he used to look at the wicked Esau.

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