148. Amīr al-mu’minīn, peace be upon him, said: Man is hidden under his tongue. 
 The meaning is that a man’s worth can be known by his speech because the speech of every person is indicative of his mind and manners, and by virtue of it his feelings and temperament can be very easily assessed. Therefore, so long as he is silent his weaknesses as well as attainments are concealed but when he speaks his real self manifests itself.
A man is hidden under his tongue; unless he speaks you cannot know his worth and value.