Stoic Philosophy and the Best of All Possible Worlds

In the letter,"On the First Cause," Seneca writes about God. I've written God with a capital G because, as a Stoic, Seneca believed in a creator god. Stoics seem to have had a curious theology. As materialists, they believed that only matter and force existed, yet believed in a soul. Consequently, they believed that the soul was material, and made of air or fire--and had a material fate when it left the body. Purified souls rose to the heavens and took their place among the stars. Impure souls disintegrated.This theology explains the Stoic interest in studying philosophy and denying the body; they wished to purify the soul so it could rise up to take its place among the stars. Of course, this summary doesn't do justice to Stoicism, and ignores differences between the beliefs of individual stoics as well as the evolution of their ideas.

In this same letter, Seneca tells Lucilius that Plato wrote that because God is good, he has created the "best of all possible worlds." I have listened to and/or read all of the Socratic Dialogues, at least those whose attribution to Plato isn't in dispute. I've listened to and read many dialogues multiple times, and have not come across this statement. Of course, I could have missed it, or it could be in one of the many disputed dialogues. 

Seneca's attribution points out a problem I first came across in Alasdair MacIntyre's writing, I believe in After Virtue, that when we talk about the Greeks, we're really talking about later interpretations, particularly the Scholastic interpretations, rather than the actual ideas of Plato or Aristotle. This passage from Seneca suggests that we may often be talking about ideas and concepts attributed to Plato et al erroneously.

I know I could have missed this statement, though I suspect it would have jumped out at me if I had come across it--so, if anyone knows where this statement is in Plato I'd love you to point it out to me.