Sa'di says

On the Qualities of Derwishes

They asked Luqman, "of whom didst thou learn manners? He replied, "From the unmannerly. Whatever I saw them do which I disapproved of, that I abstained from doing."

Not e'en in jest a playful word is said,
But to the wise, 'twill prove a fruitful theme.
To fools, a hundred chapters may be read
of grave import; to them they'll jesting seem.