The prudent man is always sincere, and feels horror at the very thought of exposing himself to the disgrace which attends upon the detection of falsehood. But though always sincere, he is not always frank and open; and though he never tells any thing but the truth, he does not always think himself bound, when not properly called upon, to tell the whole truth.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“If he ever connects himself with any society of this kind, it is merely in self-defence, not with a view to impose upon the public, but to hinder the public from being imposed upon, to his disadvantage, by the clamours, the whispers, or the intrigues, either of that particular society, or of some other of the same kind. The prudent man is always sincere, and feels horror at the very thought of exposing himself to the disgrace which attends upon the detection of falsehood. But though always sincere, he is not always frank and open; and though he never tells any thing but the truth, he does not always think himself bound, when not properly called upon, to tell the whole truth. As he is cautious in his actions, so he is reserved in his speech; and never rashly or unnecessarily obtrudes his opinion concerning either things or persons. The prudent man, though not always distinguished by the most exquisite sensibility, is always very capable of friendship.” source