This is not a law, just what the AMA states in it's rules:
Academy of Model Aeronautics National Model Aircraft Safety Code
"RC night flying requires a lighting system providing the pilot with a clear view of the model’s attitude and orientation at all times. Hand-held illumination systems are inadequate for night flying operations."
https://www.modelaircraft.org/files/105.PDF
Only thing the FAA states as far as I know is that model aircraft flights should be kept below 400 feet AGL, should be flown a sufficient distance from populated areas and full scale aircraft, and are not used for business purposes.
That's interesting stuff. It was neat to see, even though I'm not into R/C flying, but neat nonetheless.