Saving formula

I've read that once a baby starts drinking from a formula bottle it has to be thrown out after an hour if it's not all finished. I've been doing that because I'm scared of bacteria getting in there. But I was wondering if anyone keeps the bottles longer? Like if your baby only drinks 3 ounces of a 5 ounce bottle, can I put it in the fridge and use it at the next feeding? I hate wasting it! But I also don't want to worry about bacteria...any thoughts?