I can tell you where but i'll let others go farther.

It's in the Republic as part of Plato's attack on Homer and Hesiod (given the strong role epic poetry played in Greek culture Plato and philosophy would be in a sense competing with them). I'm not an expert on the subject and don't want to get too far into it since plato's doing something multifacited and interesting here and i don't want to misrepresent my reading as something strongly canonical so i'll link to perhaps the best legitimate general philosophy source on the free web: http://plato.stanford.edu/entries/plato-rhetoric/#RepBooIIIIIX

Here plato is drawing on the work of the presocratic xenophanes who also criticized the gods of homer and hesiod.