If thou hast not Sense enough to speak, have Wit enough to hold thy tongue.

Thomas Fuller (1654-1734) English physician, preacher, aphorist, writer

Introductio ad Prudentiam, Vol. 1, # 382 (1725)
    (Source)

The basic theme here is a common one. See also Twain (spurious), the Bible, Franklin, Thomas a Kempis, and Wilson.