DREAM: It is a fool’s prerogative to utter truths that no one else will speak.

Neil Gaiman (b. 1960) British author, screenwriter, fabulist

Sandman, Book 3. Dream Country, # 19 “A Midsummer Night’s Dream” (1990)
    (Source)

Because the story includes William Shakespeare as a character, and is named after Shakespeare's play (which is performed in the story), this line is sometimes misattributed to Shakespeare himself.

See also this later comment by Dream.