Aesop's Fables (1884)
14. The Quack Frog.
A Frog once made proclamation to all the beasts that he was a learned physician, and able to heal all diseases. A Fox asked him: "How can you pretend to prescribe for others, and you are unable to heal your own lame gait and wrinkled skin?"
Those who pretend that they can mend others should first mend themselves, and then they will be more readily believed.
Aesop's Fables: A New Revised Version From Original Sources (translator not identified), 1884 . Illustrations by Ernest Henry Griset (1844-1907), John Tenniel (1820-1914) and Harrison Weir (1824-1906). Available online at Project Gutenberg.