Searching through http://www.gutenberg.org/files/18947/18947-h/18947-h.htm, I don't see any explicit mention of the hammer flying back, but Snorri does say "it would never fly so far that it did not return to his hand", and I can't see how else Thor could've retrieved it after this:
It is said that the giant Hymer changed hue and grew pale from fear when he saw the serpent and beheld the water flowing into the boat; but just at the moment when Thor grasped the hammer and lifted it in the air, the giant fumbled for his fishing-knife and cut off Thor’s line at the gunwale, whereby the serpent sank back into the sea. Thor threw the hammer after it, and it is even said that he struck off his head at the bottom, but I think the truth is that the Midgard-serpent still lives and lies in the ocean.
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