Plutarch, Life of Alexander 18.2-4:
καὶ Γόρδιον πόλιν, ἑστίαν Μίδου τοῦ παλαιοῦ γενέσθαι λεγομένην, παραλαβών, τὴν θρυλουμένην ἅμαξαν εἶδε, φλοιῷ κρανείας ἐνδεδεμένην, καὶ λόγον ἐπ' αὐτῇ πιστευόμενον ὑπὸ τῶν βαρβάρων ἤκουσεν, ὡς τῷ λύσαντι τὸν δεσμὸν εἵμαρται βασιλεῖ γενέσθαι τῆς οἰκουμένης. οἱ μὲν οὖν πολλοί φασι, τῶν δεσμῶν τυφλὰς ἐχόντων τὰς ἀρχὰς καὶ δι' ἀλλήλων πολλάκις σκολιοῖς ἑλιγμοῖς ὑποφερομένων, τὸν Ἀλέξανδρον ἀμηχανοῦντα λῦσαι, διατεμεῖν τῇ μαχαίρᾳ τὸ σύναμμα, καὶ πολλὰς ἐξ αὐτοῦ κοπέντος ἀρχὰς φανῆναι. Ἀριστόβουλος δὲ καὶ πάνυ λέγει ῥᾳδίαν αὐτῷ γενέσθαι τὴν λύσιν, ἐξελόντι τοῦ ῥυμοῦ τὸν ἕστορα καλούμενον, ᾧ συνείχετο τὸ ζυγόδεσμον, εἶθ' οὕτως ὑφελκύσαντι τὸν ζυγόν.— "But Aristoboulos says the solution came quite easily to him when he lifted the peg (called the hestor) by which the yoke-band was tied to the pole off the pole, and thus loosened the yoke." That is, Alexander was thus able to expose the two ends of the knot (τῶν δεσμῶν τυφλὰς ἐχόντων τὰς ἀρχὰς) and untie it, so he didn't have to use his sword.