Глупый болтает, а умный думает

A clever man thinks before giving his opinion, whereas a fool just bobbles. See Дурак кричит, умный молчит (Д)

Cf:

A fool talks while a wise man thinks (Am.). Knowledge talks lowly; ignorance talks loudly (Am.). The less people think, the more they talk (Br.). People who know little talk much; people who know much talk little (Am.). The wise man has long ears and a short tongue (Br.). Wise men have their mouth in their heart, fools their heart in their mouth (Br.). Wise men silent, fools talk (Br.)