Thirukkural – 331
Idiocy of believing ephemeral things as enduring
is the worst of them all.
நில்லா தவற்றை நிலையின வென்றுணரும்
புல்லறி வாண்மை கடை.
This couplet is under the chapter Impermanence. Everything in this world is ephemeral. They will vanish. The idiocy of believing them as enduring is the worst idiocy of them all.