There isn't any conclusive evidence on whether or not this is safe, but some experts (and lots of moms) say it's okay to stick an unfinished bottle back in the fridge to reheat and reuse at the next feeding. (If baby doesn’t drink it all at the next feeding, toss it.)
The general guideline is: Don't leave an unfinished bottle sitting around at room temperature for more than 30 minutes to an hour, if at all. (But there isn't definitive research on the safety of this either.)
If you're finding that you have leftover milk on a regular basis, take this as a cue to make smaller bottles.