When Jacob passed to come into the land of Canaan, Esau came to him from Mount Seir in violent anger, contriving to slay him, as it is said, "The wicked plotteth against the just and gnasheth upon him with his teeth." (Ps. xxxvii. 12). Esau said: I will not slay Jacob with bow and arrows, but with my mouth and with my teeth will I slay him, and suck his blood, as it is said, "And Esau ran to meet him, and embraced him, and fell on his neck, and kissed him; and they wept" (Gen. xxxiii. 4). Do not read vayishakehu (and he kissed him) but (read) vayishkehu (and he bit him). But Jacob's neck became like ivory, and concerning him the Scripture says, "Thy neck is like the tower of ivory" (Cant. vii. 4). The wicked (Esau's) teeth became blunt, and when the wicked one saw that the desire of his heart was not realized he began to be angry, and to gnash with his teeth, as it is said, "The wicked shall see it, and be grieved; he shall gnash with his teeth and melt away" (Ps. cxii. 10).
Pirke de Rabbi Eliezer (trans. Gerald Friedlander, p285)