Uncooked rice can contain spores of Bacillus cereus, a bacterium that can cause food poisoning. The spores can survive when rice is cooked.

If rice is left standing at room temperature, the spores can grow into bacteria. These bacteria will multiply and may produce toxins (poisons) that cause vomiting or diarrhoea.

The longer cooked rice is left at room temperature, the more likely it is that the bacteria or toxins could make the rice unsafe to eat.Tips on serving rice safely
Ideally, serve rice as soon as it has been cooked.
If that isn't possible, cool the rice as quickly as possible (ideally within 1 hour).
Keep rice in the fridge for no more than 1 day until reheating.
When you reheat any rice, always check the dish is steaming hot all the way through.
Do not reheat rice more than once.