Kural - 834

The sacred law he reads and learns, to other men expounds,-
Himself obeys not; where can greater fool be found

Explanation
There are no greater fools than he who, though he has read and understood (a great deal) and even taught it to others, does not walk according to his own teaching.

Tamil Transliteration
Odhi Unarndhum Pirarkkuraiththum Thaanatangaap
Pedhaiyin Pedhaiyaar Il.