Midrasz do Rodzaju 32:82

Again the angel said to him: "Let me go" (Gen. 32:26). Jacob answered him: I will not let thee go until thou hast blessed me; and he blessed him, as it is said, "And he blessed him there" (Gen. 32:29). Again he said to him: "Let me go" (Gen. 32:26). He answered him: I will not let thee go until thou tellest me what thy name is. And (the angel) called his name Israel like his own name, for his own name was called Israel. Jacob wished to prevail over the angel, and to throw him down upon the earth. What did the angel do? He took hold of the sinew of the hip, which was upon the hollow of Jacob's thigh, and he lifted the sinew of his hip (out of its place), and it became like the fat of the dead. Therefore the children of Israel are forbidden to eat of the sinew of the hip which is upon the hollow of the animal's thigh, as it is said, "Therefore the children of Israel eat not the sinew of the hip which is upon the hollow of the thigh" (Gen. 32:82).