“Teach your tongue to say "I do not know" and you will progress.”

Source: Change of Heart

Charles Manson's first prison interview https://www.youtube.com/watch?v=JbW0agGFv88 by 60 Minutes Australia (1981)

An Old Man's Thoughts on Many Things, Of Education I
Context: I have said nothing about religious teaching as one of the means of forming a good character.... I, who am not a teacher of religion, do not presume to say how it should be taught, so taught as to be practical. If you merely teach dogmas dogmatically, you are not teaching in the sense in which I understand teaching... and learning... does not consist merely in knowing: it is not learning unless there is some corresponding doing.

Source: The Story Sisters

Source: Dreamfever

Verse 41.
To Demonicus
Context: Always when you are about to say anything, first weigh it in your mind; for with many the tongue outruns the thought. Let there be but two occasions for speech — when the subject is one which you thoroughly know and when it one on which you are compelled to speak. On these occasions alone is speech better than silence; on all others, it is better to be silent than to speak.

Life and Letters, 2:389.

In conversation, attributed by James E. McEldowney http://people.virginia.edu/~pm9k/jem/words/gandhi.html <br class="br">Posthumous publications (1950s and later)