A foolish man speaks foolishness.

[Μῶρα γὰρ μῶρος λέγει.]

Euripides (485?-406? BC) Greek tragic dramatist

Bacchæ [Βάκχαι], l. 369 [Tiresias/Τειρεσίας] (405 BC) [tr. Wodhull (1809)]
    (Source)

To Cadmus, about his grandson, Pentheus. (Source (Greek)). Alternate translations:

Folly issues from the mouth of fools.
[tr. Buckley (1850)]

Fools still speak folly.
[tr. Milman (1865)]

Fools blurt their folly out.
[tr. Rogers (1872), l. 357]

The words of a fool are folly.
[tr. Coleridge (1891)]

Fools alone speak folly.
[tr. Way (1898)]

Blind words and a blind heart.
[tr. Murray (1902)]

The words of fools finish in folly.
[tr. Arrowsmith (1960)]

He who speaks folly is himself a fool.
[tr. Kirk (1970)]

The things he has said reveal the depth of his folly.
[tr. Vellacott (1973)]

It is a fool who folly speaks.
[tr. Neuburg (1988)]

You can tell a dangerous fool by his own words.
[tr. Cacoyannis (1982)]

For a fool speaks folly.
[tr. Blessington (1993)]

For Pentheus is a fool and says foolish things.
[tr. Esposito (1998)]

He who speaks foolishness is a fool.
[tr. Woodruff (1999)]

The fool speaks foolish things.
[tr. Gibbons/Segal (2000), l. 435]

His talk is folly and he's a fool.
[tr. Kovacs (2002)]

Often a fool speaks foolishly.
[tr. Valerie (2005)]

A man who's mad tends to utter madness.
[tr. Johnston (2008)]

His foolish words will end in folly.
[tr. Robertson (2014)]

A fool says foolish things.
[tr. @sentantiq (2016)]

The speech of the fool is foolish.
[tr. @sentantiq (2018)]

The tongue of a fool makes a foolish noise.
[tr. Behr/Foster (2019)]

For a foolish man says foolish things.
[tr. Buckley/Sens/Nagy (2020)]