“Of all possessions a friend is the most precious.”

As quoted in Giordano Bruno : His Life and Thought (1950) by Dorothea Waley Singer http://www.positiveatheism.org/hist/bruno03.htm#CH3
Context: If all things are in common among friends, the most precious is Wisdom. What can Juno give which thou canst not receive from Wisdom? What mayest thou admire in Venus which thou mayest not also contemplate in Wisdom? Her beauty is not small, for the lord of all things taketh delight in her. Her I have loved and diligently sought from my youth up.

Source: How to Stop Worrying and Start Living

The Devil's Dictionary (1911)

Review of Historic Survey of German Poetry, interspersed with Various Translations by W. Taylor, in The Edinburgh Review Vol. LIII (1831), p. 178.
1830s
Context: A man's honest, earnest opinion is the most precious of all he possesses: let him communicate this, if he is to communicate anything. There is, doubtless a time to speak, and a time to keep silence; yet Fontenelle's celebrated aphorism, I might have my hand full of truth, and would open only my little finger, may be practiced to excess, and the little finger itself kept closed. That reserve, and knowing silence, long so universal among us, is less the fruit of active benevolence, of philosophic tolerance, than of indifference and weak conviction. Honest Scepticism, honest Atheism, is better than that withered lifeless Dilettantism and amateur Eclecticism, which merely toys with all opinions; or than that wicked Machiavelism, which in thought denying every thing, except that Power is Power, in words, for its own wise purposes, loudly believes every thing: of both which miserable habitudes the day, even in England, is wellnigh over.

1976

Indestructible Spirit Conference at La Paz, UFW Headquarters in Keene, California (11 January 1991)

Misattributed