Diodorus Siculus 10.18.2–6
“Because his country was a tyranny was ruled harshly under Nearchus, he set up a conspiracy against the tyrant. But when he was discovered and was compelled by Nearchus under torture to divulge who was aiding him, he said, “if only I were as in control of my body as I were my tongue.” The tyrant attacked him even more with torture for this, but Zeno withstood it for some time.
After this exchange, because he wanted to be freed from the torture and also to pay back Nearchus, he made the following plan. When the pain of the torture was at its greatest peak, he pretend that he was about to die because of the pain and yelled out, “Stop, I will tell you everything.” Once they stopped, he asked him to come close to hear what he said alone since many of the things which were about to be said he would prefer to keep secret.
When the tyrant approached happily and brought his ear near Zeno’s mouth, Zeno grabbed the tyrant’s ear with his teeth and clamped down. Even though his attendants rushed up and were serving up everything kind of pain to the tortured man to loosen the bite, he sank his teeth in further. Finally, when they could not conquer the man’s bravery, they stabbed him until he released his teeth. By this plan he found freedom from his pains and obtained what payback their was from the tyrant.”
 ὅτι τυραννουμένης τῆς πατρίδος ὑπὸ Νεάρχου σκληρῶς, ἐπιβουλὴν κατὰ τοῦ τυράννου συνεστήσατο. καταφανὴς δὲ γενόμενος, καὶ κατὰ τὰς ἐν ταῖς βασάνοις ἀνάγκας διερωτώμενος ὑπὸ τοῦ Νεάρχου τίνες ἦσαν οἱ συνειδότες, ὤφελον γάρ, ἔφησεν, ὥσπερ τῆς γλώττης εἰμὶ κύριος, οὕτως ὑπῆρχον καὶ τοῦ σώματος.  τοῦ δὲ τυράννου πολὺ μᾶλλον ταῖς βασάνοις προσεπιτείναντος, ὁ Ζήνων μέχρι μέν τινος διεκαρτέρει· μετὰ δὲ ταῦτα σπεύδων ἀπολυθῆναί ποτε τῆς ἀνάγκης καὶ ἅμα τιμωρήσασθαι τὸν Νέαρχον, ἐπενοήσατό τι τοιοῦτον.  κατὰ τὴν ἐπιτονωτάτην ἐπίτασιν τῆς βασάνου προσποιηθεὶς ἐνδιδόναι τὴν ψυχὴν ταῖς ἀλγηδόσιν ἀνέκραγεν, ἄνετε, ἐρῶ γὰρ πᾶσαν ἀλήθειαν. ὡς δ’ ἀνῆκαν, ἠξίωσεν αὐτὸν ἀκοῦσαι κατ’ ἰδίαν προσελθόντα· πολλὰ γὰρ εἶναι τῶν λέγεσθαι μελλόντων ἃ συνοίσει τηρεῖν ἐν ἀπορρήτῳ.  τοῦ δὲ τυράννου προσελθόντος ἀσμένως καὶ τὴν ἀκοὴν τῷ στόματι παραβαλόντος, ὁ Ζήνων τοῦ δυνάστου περιχανὼν τὸ οὖς ἐνέπρισε τοῖς ὀδοῦσι. τῶν δὲ ὑπηρετῶν ταχὺ προσδραμόντων, καὶ πᾶσαν τῷ βασανιζομένῳ προσφερόντων τιμωρίαν εἰς τὸ χαλάσαι τὸ δῆγμα, πολὺ μᾶλλον προσενεφύετο.  τέλος δ’ οὐ δυνάμενοι τἀνδρὸς νικῆσαι τὴν εὐψυχίαν, παρεκέντησαν αὐτὸν ἵνα διίῃ τοὺς ὀδόντας. καὶ τοιούτῳ τεχνήματι τῶν ἀλγηδόνων ἀπελύθη καὶ παρὰ τοῦ τυράννου τὴν ἐνδεχομένην ἔλαβε τιμωρίαν.
In the other version of this story, Zeno of Elea bites off his own tongue
From the Suda
“Zeno, the son of Teleutagoros, Elean, one of the philosophers who lived in the same time as Pythagoras and Democritus during the 78th Olympiad. He was a student of Xenophanes or Parmenides. He wrote Disagreements, and Explanation of Empedokles and Against the Philosophers on Nature. They say that he invented dialectic, and that Empedokles invented rhetoric. Some say that he was caught trying to kill the tyrant Nearkhos—although some say it was Diomedon. When he was being questioned by him, he bit down on his own tongue, cut it off, and spat it at the Tyrant. Then he was thrown into a mortar and ground down into a mush.”
Ζήνων, Τελευταγόρου, Ἐλεάτης, φιλόσοφος τῶν ἐγγιζόντων Πυθαγόρᾳ καὶ Δημοκρίτῳ κατὰ τοὺς χρόνους, ἦν γὰρ ἐπὶ τῆς οη# Ὀλυμπιάδος, μαθητὴς Ξενοφάνους ἢ Παρμενίδου. ἔγραψεν Ἔριδας, Ἐξήγησιν τῶν Ἐμπεδοκλέους, Πρὸς τοὺς φιλοσόφους περὶ φύσεως. τοῦτόν φασιν εὑρετὴν εἶναι τῆς διαλεκτικῆς, ὡς Ἐμπεδοκλέα τῆς ῥητορικῆς. καθελεῖν δὲ θελήσας Νέαρχον, οἱ δὲ Διομέδοντα, τὸν Ἐλέας τύραννον, ἑάλω. καὶ ἐρωτώμενος ὑπ’ αὐτοῦ τὴν γλῶτταν αὑτοῦ ἐνδακὼν καὶ ἀποτεμὼν προσέπτυσε τῷ τυράννῳ. καὶ ἐν ὅλμῳ βληθεὶς συνετρίβη πτισσόμενος.
Diogenes Laertius tells the same story as Diodorus:
Diogenes Laertius, Lives of the Philosophers 9.26
“When he was interrogated about his conspirators and the arms he was bringing to Liparas, he informed on all the friends of the tyrant because he wished to isolate that man. Then, telling him that he could tell him something about these things to his ear only, he bit down on [Nearchus’] ear and would not let go until he was stabbed to death, suffering the same fate as the tyrannocide Aristogeiton.”
ὅτε καὶ ἐξεταζόμενος τοὺς συνειδότας καὶ περὶ τῶν ὅπλων ὧν ἦγεν εἰς Λιπάραν, πάντας ἐμήνυσεν αὐτοῦ τοὺς φίλους, βουλόμενος αὐτὸν ἔρημον καταστῆσαι· εἶτα περί τινων εἰπεῖν ἔχειν τινα <ἔφη> αὐτῷ πρὸς τὸ οὖς καὶ δακὼν οὐκ ἀνῆκεν ἕως ἀπεκεντήθη, ταὐτὸν ᾿Αριστογείτονι τῷ τυραννοκτόνῳ παθών.
This is not the same Zeno as Zeno of Citium, who is credited with founding stoicism. Zeno from Elea is known for paradoxes!