The Latin:
Themistocles fertur Seriphio cuidam in iurgio respondisse, cum ille dixisset non eum sua, sed patriae gloria splendorem adsecutum: ‘Nec hercule,’ inquit, ‘si ego Seriphius essem, nec tu, si Atheniensis clarus umquam fuisses.’ (Cicero, De Senectute, 3.

The Loeb rendering I found:
there is a story that when, in the course of a quarrel, a certain Seriphian7 had said to Themistocles, "Your brilliant reputation is due to your country's glory, not your own," Themistocles replied, "True, by Hercules, I should never have been famous if I had been a Seriphian, nor you if you had been an Athenian."
I'm not sure about the last part - I'm more tempted to read it
Hercule, nec (si ego Seriphius essem) nec (si tu Atheniensis [esses]) (clarus umquam fuisses)
By Hercules, neither if I were a Seriphian, nor if you were an Athenian, would you ever have been famous! (i.e. it's not all down to country of birth - you're the kind of man who would be a pleb whatever the circumstances!)
I think (contextually) that, if they're quarrelling and the Seriphian is insulting Themistocles, a reading which has him basically agreeing that his glory is due to Athens is odd, and I think (textually) supplying "fuissem" to make "clarus umquam" apply to them both is maybe a stretch. Am I mad, or am I justified?