Kural - 849

That man is blind to eyes that will not see who knowledge shows;-
The blind man still in his blind fashion knows

Explanation
One who would teach a fool will (simply) betray his folly; and the fool would (still) think himself "wise in his own conceit".

Tamil Transliteration
Kaanaadhaan Kaattuvaan Thaankaanaan Kaanaadhaan
Kantaanaam Thaankanta Vaaru.