From the Food Standards Agency
Q. I've heard that reheating rice can cause food poisoning. Is this true?
A. It's true that you could get food poisoning from eating reheated rice. But it's not actually the reheating that's the problem - it's the way the rice has been stored before reheating.
Uncooked rice can contain spores of bacteria that can cause food poisoning. When the rice is cooked, the spores can survive. Then, if the rice is left standing at room temperature, the spores will multiply and may produce poisons that cause vomiting or diarrhoea. Reheating the rice won't get rid of these poisons.
So, the longer cooked rice is left at room temperature, the more likely it is that poisons produced could stop the rice being safe to eat.
It's best to serve rice when it has just been cooked. If that isn't possible, cool the rice as quickly as possible (ideally within one hour) and keep it in the fridge for no more than one day until reheating.