The prudent keep their knowledge to themselves, but a fool's heart blurts out folly. (NIV)
A prudent man is reluctant to display his knowledge, but the heart of [self-confident] fools proclaims their folly. [Isa. 32:6.] (AMP)
A prudent man concealeth knowledge: but the heart of fools proclaimeth foolishness. (KJV)