There is a popular saying:
Everything you say should be true, but not everything true should be said.
On the internet, many pages attribute this quote to Voltaire - however, I could not find any source. Did Voltaire really say or write this?
Note: I did find two related quotes:
There are truths which are not for all men, nor for all times.
Voltaire, Letter to François-Joachim de Pierre, cardinal de Bernis (23 April 1764)
[...] und wenn alles, was man sagt, wahr sein muß, so ist darum nicht auch Pflicht, alle Wahrheit öffentlich zu sagen.".
[...] and if everything one says must be true, this does not mean it is a duty to make all truth public.
Immanuel Kant, nachlass
Maybe one of these morphed into the one I am asking about?