Targum for Genesis 27:1

And it was when Izhak was old and his eyes were darkened from seeing,--because when his father was binding him he had seen the Throne of Glory, and from that time his eyes had begun to darken,--that he called Esau his elder son, on the fourteenth of Nisan, and said to him, My son, behold, this night they on high praise the Lord of the world, and the treasures of the dew are opened in it. And he said, Behold, I am.