Full Idea
He said that he did not know whether there were gods - but this is the same as saying that he knew there were no gods.
Gist of Idea
He said he didn't know whether there are gods - but this is the same as atheism
Source
comment on Protagoras (fragments/reports [c.441 BCE], A23) by Diogenes (Oen) - Wall inscription 11 Chil 2
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.219