Passage 3.15.4
προσεγένετο δὲ ἐς τοῦ πολέμου τὴν ἀρχὴν καὶ ἄλλο τοιόνδε. Οἰωνὸς ἡλικίαν μὲν μειράκιον, ἀνεψιὸς δὲ Ἡρακλεῖ---Λικυμνίου γὰρ παῖς ἦν τοῦ ἀδελφοῦ τοῦ Ἀλκμήνης---ἀφίκετο ἐς Σπάρτην ἅμα Ἡρακλεῖ· περιιόντι δὲ καὶ θεωμένῳ τὴν πόλιν, ὡς ἐγίνετο κατὰ τοῦ Ἱπποκόωντος τὴν οἰκίαν, ἐνταῦθά οἱ κύων ἐπεφέρετο οἰκουρός. ὁ δὲ τυγχάνει τε ἀφεὶς λίθον ὁ Οἰωνὸς καὶ καταβάλλει τὴν κύνα· ἐπεκθέουσιν οὖν τοῦ Ἱπποκόωντος οἱ παῖδες καὶ ῥοπάλοις τύπτοντες κατεργάζονται τὸν Οἰωνόν.
Another event occurred that also sparked the beginning of the war. Oionos, still a youth in age and cousin to Heracles—for he was the son of Likymnios, the brother of Alcmene—had come to Sparta accompanying Heracles. While wandering around and examining the town, he happened to come to the house of Hippocoön, where a watchdog ran out at him. Oionos immediately threw a stone and killed the dog. Then Hippocoön's sons rushed forth and, striking him with clubs, caused Oionos' death.