Wittgenstein: "Whereof one cannot speak, one must remain silent" (Tractatus, 1922).
Diderot: "True philosophy would find itself considerably briefer if all philosophers would be willing, like me, to abstain from speaking of what is manifestly incomprehensible" (Encyclopedia, "To Act").
I am far from an expert on matters of philosophy, especially across different historical eras. But, as noted in a previous posting (here), the Wittgenstein quote has always appeared to me to be nothing more than a confounding tautology. The Diderot quote, by contrast, strikes me as sensible (and in his historical context, of course, religiously subversive). Perhaps Wittgenstein meant to say something similar to what Diderot asserted (more clearly)?
By the way, I found the Diderot quote in Arthur M. Wilson's excellent, eponymously entitled 1972 biography (p. 139).