“The silence of a wise man is always meaningful.”

The Sayings of the Wise (1555)

Of Bashfulness
Bartlett's Familiar Quotations, 10th ed. (1919)

Book V, chapter 15, section 43; translated by Andrew P. Peabody
Tusculanae Disputationes – Tusculan Disputations (45 BC)
Context: Now since perturbations of mind create misery, while quietness of mind makes life happy, and since there are two kinds of perturbations, grief and fear having their scope in imagined evils, inordinate joy and desire in mistaken notions of the good, all being repugnant to wise counsel and reason, will you hesitate to call him happy whom you see relieved, released, free from these excitements so oppressive, and so at variance and divided among themselves? Indeed one thus disposed is always happy. Therefore the wise man is always happy.

Unidentified Plays, Fragment 977
Variant translation: Silence is true wisdom's best reply. (See Discussion page for sourcing information)

Introductio ad prudentiam: Part II (1727)

Le silence est l'esprit des sots
Et l'une des vertus du sage.
Bernard de Bonnard, "Le Silence," http://books.google.com/books?id=9gAvAAAAMAAJ&pg=PR14&dq=%22Et+l%27une+des+vertus+du+sage%22+Bonnard&ei=iyzvR-bFOIa4zASV0PyoBQ#PPA244,M1 L'Almanach des Muses (1776)
Misattributed

Source: Sweetest song I know

Source: The Angel's Game