In American English it is standard to use an article in front of hospital. In British English I believe "Into hospital" is more commonly used, but I've also seen" in hospital". I tried googling "he went into hospital" and "he went in hospital" and got hits for both. Here's a dictionary entry that may help: http://www.macmillandictionary.com/dictionary/british/hospital

I went into the hospital for a operation.

I went into the hospital for an operation. I went in a hospital for an operation. and, what I think is more right.... I went to the hospital for an operation.