Hercules Furens 49, 55-63 (c.65 AD)
Heracles parades Cerberus through Greece as he takes him back to Eurystheus.
49:
parum est reverti, foedus umbrarum perit:
55-63:
patefacta ab imis manibus retro via est
et sacra dirae mortis in aperto iacent.
at ille, rupto carcere umbrarum ferox,
de me triumphat et superbifica manu
atrum per urbes ducit Argolicas canem.
viso labantem Cerbero vidi diem
pavidumque Solem; me quoque invasit tremor,
et terna monstri colla devicti intuens
timui imperasse.
DSS no. 42. Translation can be found here.