Known sourcecanonical
"He [Pliny the Elder] used to say that "no book was so bad but some good might be got out of it.""
Pliny the Younger / Letters. Book iii. Letter v. 10.
Bartlett's Familiar Quotations, 9th ed. (Little, Brown, 1905), public domain
Bartlett's Familiar Quotations, 9th ed. (Little, Brown, 1905), public domain