Are you thinking of Gorgias? We don't have that text anymore, just people discussing it, and there's some debate on whether it was something he believed or if it was just a mind exercise.

You could be thinking of "the only thing I know is that I know nothing" which is commonly believed to have been said by Socrates, but it's really not the case (though a bit complicated) https://en.wikipedia.org/wiki/I_know_that_I_know_nothing

You might have luck at r/askphilosophy/ as well