I will now say something because I have to say something.
What you have here is an elliptical sentence. That omission/ellipsis is quite common in speech and informal writing.
This is what the Chicago Manual of Style Guide (17th ed.) says
A grammatical ellipsis (sometimes called an omission) occurs when part of a clause is left understood and the reader or listener is able to supply the missing words. This “recovery” of omitted words is possible because of shared idiomatic knowledge, context, and what’s called the principle of recoverability
- {he preferred chocolate, she vanilla [preferred is understood in the second clause]}.
A sentence containing such an ellipsis is called an elliptical sentence.
In your case, the listener is expecting a contrast after "Wise men speak ..." and so "speak" is readily understood and implied even when it is omitted.
I got more to say. We can also use commas to indicate elision (yours is a case where a comma is not needed)
A comma is often used to indicate the omission of a word or words readily understood from the context.
- In Illinois there are seventeen such schools; in Ohio, twenty; in Indiana, thirteen.
- Thousands rushed to serve him in victory; in defeat, none.
The comma may be omitted if the elliptical construction is clear without it.
- One student excels at composition, another at mathematics, and the third at sports.
- Jasper missed her and she him.
@CompuChip said something in the comments, which makes a whole lot of sense. Even though you don't need a comma after "fools", a pause after "fools" is preferred. A pause there creates a somewhat dramatic impact ... and also aids clarity.