Once, a long time ago, The Riddler made the mistake of telling me that. It means as long as he dishes out riddles, he's... He's not finished with his plot.
10 seconds sound clip from Batman (1966) - A Riddle a Day Keeps the Riddler Away (S1E11) TV Series.
You can hear this line at 00:10:34 in the Blu-ray version of the movie.
Quote context
[...]
- If you figure it out, look for another royal riddle in this crown.
- That ends any chance of tracking him with the homing device. Shall we after him in the manhole?
- We'll never find him. There're hundreds of tunnels down here.
- A riddle a day keeps The Riddler away.
- What?
- When the riddles stop, that's when he's ready for his final caper.
- That diamond tiara wasn't what he was really after.
- Right, Robin. But we have two more riddles to tell us what it may be.
- Now. First clue was... What room can no one enter?
[...]