Nor ought you ever to laugh at any joke or smart saying of your own; for you will be thought to applaud your own wit. It belongs to the company, and not to him who says a good thing, to express their approbation by a laugh.

[Né de’ tuoi medesimi motti voglio che tu ti rida, che è un lodarti da te stesso: egli tocca di ridere a chi ode, e non a chi dice.]

Giovanni della Casa (1503-1556) Florentine poet, author, diplomat, bishop

Galateo: Or, A Treatise on Politeness and Delicacy of Manners [Il Galateo overo de’ costumi], ch. 30 (1558) [tr. Graves (1774)]
    (Source)

(Source (Italian)). Alternate translations:

But a man must beware he doe not laughe at his owne gestes, and his doings. For that makes men weene hee woulde faine praise him selfe. It is for other men to laughe that heare, and not for him that telles the tale.
[tr. Peterson (1576)]

And I do not want you to laugh at your own jokes, for it is a type of self-praise. It is the hearer who should laugh, not the speaker.
[tr. Eisenbichler/Bartlett (1986)]

The question of laughter lies with the hearer, not with the narrator.
[Source]