Apollodorus Labor 12
[2.5.12] A twelfth labour imposed on Hercules was to bring Cerberus from Hades. Now this Cerberus had three heads of dogs, the tail of a dragon, and on his back the heads of all sorts of snakes. When Hercules was about to depart to fetch him, he went to Eumolpus at Eleusis, wishing to be initiated. However it was not then lawful for foreigners to be initiated: since he proposed to be initiated as the adoptive son of Pylius. But not being able to see the mysteries because he had not been cleansed of the slaughter of the centaurs, he was cleansed by Eumolpus and then initiated. And having come to Taenarum in Laconia, where is the mouth of the descent to Hades, he descended through it. But when the souls saw him, they fled, save Meleager and the Gorgon Medusa. And Hercules drew his sword against the Gorgon, as if she were alive, but he learned from Hermes that she was an empty phantom. And being come near to the gates of Hades he found Theseus and Pirithous, him who wooed Persephone in wedlock and was therefore bound fast. And when they beheld Hercules, they stretched out their hands as if they should be raised from the dead by his might. And Theseus, indeed, he took by the hand and raised up, but when he would have brought up Pirithous, the earth quaked and he let ho. And he rolled away also the stone of Ascalaphus. And wishing to provide the souls with blood, he slaughtered one of the kine of Hades. But Menoetes, son of Ceuthonymus, who tended the kine, challenged Hercules to wrestle, and being seized round the middle, had his ribs broken; howbeit, he was let off at the request of Persephone. When Hercules asked Pluto for Cerberus, Pluto ordered him to take the animal provided he mastered him without the use of the weapons which he carried. Hercules found him at the gates of Acheron, and cased in his cuirass and covered by the lion's skin, he flung his arms round the head of the brute, and though the dragon in its tail bit him, he never relaxed his grip and pressure till it yielded. So he carried it off and ascended through Troezen. But Demeter turned Ascalaphus into a short-eared owl, and Hercules, after showing Cerberus to Eurystheus, carried him back to Hades.
Source: Loeb Apollodorus, translated by Sir James G. Frazer, 1921.