Apollodorus Library 1.9.22

[22]
  Being rid of the Harpies, Phineus revealed to the Argonauts the course of their voyage, and advised them about the Clashing Rocks ¹ in the sea. These were huge cliffs, which, dashed together by the force of the winds, closed the sea passage. Thick was the mist that swept over them, and loud the crash, and it was impossible for even the birds to pass between them. So he told them to let fly a dove between the rocks, and, if they saw it pass safe through, to thread the narrows with an easy mind, but if they saw it perish, then not to force a passage. When they heard that, they put to sea, and on nearing the rocks let fly a dove from the prow, and as she flew the clash of the rocks nipped off the tip of her tail. So, waiting till the rocks had recoiled, with hard rowing and the help of Hera, they passed through, the extremity of the ship's ornamented poop being shorn away right round. Henceforth the Clashing Rocks stood still; for it was fated that, so soon as a ship had made the passage, they should come to rest completely.