Apollodorus Library 2.7.3

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  After the capture of Elis he marched against Pylus, ¹ and having taken the city he slew Periclymenus, the most valiant of the sons of Neleus, who used to change his shape in battle. ² And he slew Neleus and his sons, except Nestor; for he was a youth and was being brought up among the Gerenians. In the fight he also wounded Hades, who was siding with the Pylians. ³
  Having taken Pylus he marched against Lacedaemon, wishing to punish the sons of Hippocoon, ⁴ for he was angry with them, both because they fought for Neleus, and still angrier because they had killed the son of Licymnius. For when he was looking at the palace of Hippocoon, a hound of the Molossian breed ran out and rushed at him, and he threw a stone and hit the dog, whereupon the Hippocoontids darted out and despatched him with blows of their cudgels. It was to avenge his death that Hercules mustered an army against the Lacedaemonians. And having come to Arcadia he begged Cepheus to join him with his sons, of whom he had twenty. But fearing lest, if he quitted Tegea, the Argives would march against it, Cepheus refused to join the expedition. But Hercules had received from Athena a lock of the Gorgon's hair in a bronze jar and gave it to Sterope, daughter of Cepheus, saying that if an army advanced against the city, she was to hold up the lock of hair thrice from the walls, and that, provided she did not look before her, the enemy would be turned to flight. ⁵ That being so, Cepheus and his sons took the field, and in the battle he and his sons perished, and besides them Iphicles, the brother of Hercules. Having killed Hippocoon and his sons and subjugated the city, Hercules restored Tyndareus and entrusted the kingdom to him.