Midrasch zu Mischlej 29:20

And Isaac trembled very exceedingly (Gen. 27:33). Scripture states elsewhere in allusion to this verse: The fear of man bringeth a snare; but whoso putteth his trust in the Lord shall be set up on high (Prov. 29:25). Because of the fear that Jacob brought upon Isaac, it would have been fitting for him to have cursed him. Who compelled him to bless Jacob? Whoso putteth his trust in the Lord shall be set up on high. R. Levi said in the name of R. Hama the son of Hanina: Isaac was terrified twice, once when he was bound upon the altar, and again when Esau entered. There is no way of knowing which event frightened him more, but since the word exceedingly is mentioned in this verse, you may presume that this was the more terrifying experience. The Holy One, blessed be He, said to Israel: Because of the evil inclination, you do not pursue My ways in this world, but in the future I will place within you a new heart; And I will put a new spirit within you; and I will remove the stony heart out of your flesh, and will give you a heart of flesh (Ezek. 11:19).