Earliest attribution found in Who Said That?: More than 2,500 Usable Quotes and Illustrations https://books.google.nl/books?id=7mn8AwAAQBAJ&pg=PT63 (1995) by George Sweeting. Online sources always attribute the quote to Augustine, but never specify in which of his works it is to be found.
Disputed
“Believe what you like, but don't believeyou read without questioning it.”
Source: Questionable Creatures: A Bestiary
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Pauline Baynes 1
English illustrator of children's books 1922–2008Related quotes
“Don't believe everything you read on the Internet.”
This quote is frequently purposefully misattributed to Lincoln or others long dead before the age of the internet in order to emphasize its point using humour; not all such attributions, or other claims, found on the Internet are as obviously flawed. " "Cite and sound: the pleasures and pitfalls of quoting people", by Tom Calverley, The Guardian (14 October 2014) http://www.theguardian.com/media/mind-your-language/2014/oct/14/mind-your-language-quotations
Variations:
Don't believe everything you read online.
Don't trust everything you see on the Internet.
Everything you read on the Internet is true.
The trouble with quotes on the internet is that you never know whether or not they're genuine.
Misattributed
“Books are like truth serum-- if you don't read, you can't figure out what's real.”
Source: Freak the Mighty
“You don't know what I'm all about / Like killing cops and reading Kerouac”
"Boxcar"
24 Hour Revenge Therapy (1993)
Romulus thumps his chest. "Honor is what you do."
Source: Morning Star (2016), Ch. 42: The Poet