From The History of Herodotus (book IX), in the translation given by Lawrence Norfolk in In the Shape of a Boar, footnote 103, page 29:
[Although Hegesistratus was] made fast in iron stocks, he got an iron weapon which was brought by some means into his prison, and straightway conceived a plan of such courage as we have never known; reckoning how best the rest of it might get free, he cut off his own foot at the instep. This done, he tunneled through the wall out of the way of the guards who kept watch over him, and so escaped to Tegea. All night he journeyed, and all day he hid and lay concealed in the woods, till on the third night he came to Tegea, while all the people of Lacedaemon sought him.