Hi!
Whatever cooked food, not just spinach, is NOT to be given to rabbits.
Raw spinash is okay to give ONLY in minimum quantities once or twice per week. The reason is that spinach (and kale) contain oxalates, which are toxic to rabbits if fed over the time, i.e. it has an accumulative effect on them.
In order to be on the safe side I never feed spinach and kale to my bunny, there are so many other veggies and herbs for him to eat.
Follow Haley's proposal and read the specific thread and other library material on what to feed your rabbit.
Marietta