מדרש אויף יהושע 7:8

"And you have given me the back of your enemies." This passage speaks about Judah. Rabbi Joshua ben Levi said in the name of Rabbi Judah bar Elai that it is a legendary tradition that Judah killed Esau. When did this happen? When Isaac died, Jacob and Esau and all the tribes went to bury him, as it is written (Genesis 35:29), "And Esau and Jacob, his sons, buried him." They went into a cave and stood and wept, and the tribes stood and paid tribute to Jacob. They went out of the cave so that Jacob would not be disgusted and humiliated in front of them. Esau himself crawled into the cave. Judah went in after him and said, "Perhaps he will kill my father." Esau crawled in and found that he was seeking to kill his father. Immediately, Judah stood up and killed him from behind. Why did he not kill him face to face? Because Esau's face resembled Jacob's, so Judah honored him and killed him from behind. And this is why his father blessed him (Genesis 49:8), "Your hand shall be on the neck of your enemies." Why on the neck? Joshua argued before the Holy One, blessed be He, that He should give him the neck, but He did not give it to him. As it is written (Joshua 7:8), "What will you do for your great name when Israel turns its back?" Yet it did not help anything. To whom did He give the neck? To the tribe of Judah, as it is written, "Your hand shall be on the neck of your enemies." Similarly, David said, "And you have given me the back of my enemies" (Psalm 18:41). My interpreter said, "And this is for Judah," as it is written (Deuteronomy 33:7), "And this is for Judah," and he said, "Who taught his hands to fight for him?" From Migdal. As it is written (1 Samuel 17:49), "And the stone sank into his forehead, and he fell on his face." He did not need to fall, but after him, and why did he fall on his face? Because the angel pushed him on his face. The Holy One, blessed be He, said, "The mouth that blasphemes and reviles (may it hang in the air) will be closed in the dust." As it is written (Job 20:11), "His bones are full of his youthful vigor, but it lies down with him in the dust." Another explanation is why on his face, so that David would not be upset and go and cut off his head. He was paid twelve and a half pieces (on the side). His height was filled in front of him and behind him, six cubits and a span. Therefore, he fell on his face between the legs of David, as it is said (Psalms 110:1) "The Lord said to my lord, sit at my right hand." Another explanation is why on his face, because Dagon, his god, was troubled in his heart to fulfill what is said (Leviticus 26:30) "And I will cast your carcasses upon the carcasses of your idols." Another explanation is why on his face, to fulfill what is said (Psalms 18:41) "They cried, but there was none to save them; even unto the LORD, but He answered them not." I will destroy them; I will make the decision for their strange death, as it is said (Leviticus 25:46) "To be your property absolutely."

"and he will cause them to inherit": If he causes them to inherit, they will inherit; if not, not. And thus do we find that when they went to do battle with Ai there fell of them some thirty-six righteous men, viz. (Joshua 7:5) "And the men of Ai smote of them some thirty-six righteous men … (6) and Joshua tore his clothing and fell on his face to the ground before the ark of the L-rd until the evening … (7) and Joshua said: Ahah, L-rd, G-d, why did You bring this people across the Jordan … (8) Please, my L-rd, what shall I say now that Israel has turned its back … (10) And the L-rd said to Joshua: Arise! Why are you fallen on your face!'" Is this not what I said to Moses from the very beginning? If he passes over before them, they shall pass; if not, they shall not pass. If he causes them to inherit, they shall inherit; if not, they shall not inherit — and you sent them and did not pass over before them!

“Judah, you shall your brothers acknowledge; your hand will be at the nape of your enemies; your father’s sons will prostrate themselves to you” (Genesis 49:8).
“Judah, you shall your brothers acknowledge” – your brothers acknowledge you; your mother acknowledges you; I, myself, acknowledge you.⁴⁷All acknowledge you as worthy of honor and kingship.
Rabbi Shimon ben Yoḥai said: All your brothers will be called by your name. A person does not say: I am a Reubenite, I am a Simeonite, but rather: I am a Jew [Yehudi].⁴⁸Judah is called Yehuda in Hebrew. Rabbi Yehuda bar Simon said: This is analogous to a king who had twelve sons, and there was one who was more beloved to him than all of them, and he gave him a portion of his own and a portion with his brothers.⁴⁹Similarly, Judah was its own tribe, and additionally the entire nation became known as Jews.
“Your hand will be at the nape of your enemies” – how many times did Joshua prostrate himself so that the nape would be shown him, but it was not granted to him, as it is stated: “Please, my Lord, what can I say after [Israel] has turned [its nape before their enemies?]” (Joshua 7:8). To whom did He grant it? To David: “My enemies, you had them turn their napes to me” (II Samuel 22:41). Why? Because it was his ancestral endowment, as it is written in his regard: “Your hand will be at the nape of your enemies.”
“Your father’s sons will prostrate themselves to you” – here it says: “Your father’s sons will prostrate themselves to you,” but elsewhere, it says: “Your mother’s sons will prostrate themselves to you” (Genesis 27:29). It is that Isaac, because he took only one wife, said: “Your mother’s sons will prostrate themselves to you.” But [when] Jacob [gave blessings], because he took four wives, it is written: “Your father’s sons will prostrate themselves to you.”