Midrasz do Rodzaju 49:18

Dan shall be a serpent on the way (Gen. 49:16). All animals travel about in pairs, but the serpent goes about alone. And just as the serpent is vengeful so too was Samson, as is said: I may be this once avenged of the Philistines for my two eyes (Judg. 16:18). When Jacob saw him he exclaimed: I wait for Thy salvation, O Lord (Gen. 49:18).