Passage 1.22.5
ἀνήγετο μὲν γὰρ ἡ ναῦς μέλασιν ἱστίοις ἡ τοὺς παῖδας φέρουσα ἐς Κρήτην, Θησεὺς δὲ---ἔπλει γὰρ τόλμης τι ἔχων ἐς τὸν Μίνω καλούμενον ταῦρον---πρὸς τὸν πατέρα προεῖπε χρήσεσθαι τοῖς ἱστίοις λευκοῖς, ἢν ὀπίσω πλέῃ τοῦ ταύρου κρατήσας· τούτων λήθην ἔσχεν Ἀριάδνην ἀφῃρημένος· ἐνταῦθα Αἰγεὺς ὡς εἶδεν ἱστίοις μέλασι τὴν ναῦν κομιζομένην, οἷα τὸν παῖδα τεθνάναι δοκῶν, ἀφεὶς αὑτὸν διαφθείρεται· καί οἱ παρὰ Ἀθηναίοις ἐστὶ καλούμενον ἡρῷον Αἰγέως.
For the ship that carried the youths to Crete sailed out with black sails, but Theseus—who was voyaging with the intention of attacking the bull called the Minotaur—had arranged beforehand with his father that he would use white sails if he sailed back victorious. However, being distracted by Ariadne, Theseus forgot this. Thus, when Aegeus saw the ship returning with black sails, believing his son had perished, he threw himself down and was killed. Among the Athenians there is accordingly a sanctuary called the Heroön of Aegeus.