The riddle itself is pretty basic. A person is a piece of wood when he's a ruler.
7 seconds sound clip from Batman (1966) - A Riddle a Day Keeps the Riddler Away (S1E11) TV Series.
You can hear this line at 00:05:46 in the Blu-ray version of the movie.
Quote context
[...]
- Batman, I'm having trouble... With a riddle.
- We'll be right over. To the Batpoles.
- It's The Riddler. His modus operandi alright.
- I agree. Nobody else would use such a spectacular and bizarre method to flaunt a puzzle.
- What the devil is the culprit trying to tell us?
- The riddle itself is pretty basic. A person is a piece of wood when he's a ruler.
- You figure it has something to do with King Boris?
- Maybe.
- But no attempt was made to abduct or to harm him.
- Wood, ruler, king. Tsk. A chess king. The International Chess Tournament tonight!
- Twenty five thousand dollars in prize money.
[...]