Genesis 27:41

And Esau kept hatred in his heart against Jakob his brother, on account of the order of blessing with which his father had blessed him. And Esau said in his heart, I will not do as Kain did, who slew Habel in the life (time) of his father, for which his father begat Sheth, but will wait till the time when the days of mourning for the death of my father come, and then will I kill Jakob my brother, and will be found the killer and the heir.

AND ESAU SAID IN HIS HEART. Rabbi Abraham ibn Ezra said, “It is possible that he revealed his secret to one of his friends.”
But this is not necessarily so for any decision which a person reaches after deliberation is referred to in Hebrew as “speaking with the heart,” even when it also includes speech with the lips. A similar case is found in the verse, And with a double heart do they speak, for the will is synonymous with the heart. And all Israel were of one heart to make David king, meaning that they had all arrived at the same decision and were talking of it. Similarly, And Jeroboam said in his heart., (Verse 28). Here also there was speech which Scripture had referred to as being said in his heart. Similarly did Eliezer say, And before I had finished speaking in my heart, whereas the prayer there was with his lips, as Scripture states, And he said, O Eternal G-d of my lord Abraham., Verse 12. However it is possible that the expression, before I had finished speaking in my heart, means “before I had concluded the thought in my mind.”
Now here the verse states that when the great hatred of Jacob permeated Esau’s soul because of the blessing, he conceived the idea of murdering his brother, and with this he comforted himself from his depressed state. This explains the verse, And the words of Esau were told to Rebekah. This is why she said to Jacob, Thy brother Esau comforts himself to slay thee.
Now Rashi comments, “She was told, through Ruach Hakodesh, what Esau was thinking in his heart.” And so it is found in Bereshith Rabbah. If so, Esau’s words and thoughts were only in his heart, just as in the verse, I spoke with my own heart.
The reason that Esau said, Let the days of mourning for my father be at hand then will I slay my brother is because he would not bring grief to his father during his lifetime. Perhaps it was due to his fear lest his father curse him, and his blessing would then turn into a curse. Rebekah, [who nevertheless advised Jacob to flee], feared that perhaps the elderly one would die suddenly, and Esau would then kill Jacob, or perhaps he might find occasion to kill him even during Isaac’s lifetime.

וישטום...יקרבו ימי אבל אבי, he wished fervently for the day of his father’s death which was expected soon as else why did he already bless his sons? He planned to kill Yaakov after the mourning period after his father would have passed. He did not ant to kill Yaakov while their father was still alive, so as not to be responsible for Yitzchok dying as a result of grief over having lost his son.

יקרבו ימי אבל אבי, “the time of mourning for the passing of my father will come about soon, etc.” Esau means that if he were to bring about his father’s premature death by murdering Yaakov, he would have accomplished nothing, as his father would then die prematurely through grieving over the loss of his son Yaakov. This is why he will await his father’s death from natural causes before carrying out his plan to kill his brother.