About ripe bananas for banana bread
Cut or mashed bananas, like many fruit, will turn brown after a very short time, a few minutes should show first discoloration and after a few hours, the surface will be brown. This is primarily a matter of aesthetics, because the fruit looks less appealing.

As far as food safety is concerned, it depends on your definition of “a couple of days”. The rule of thumb for storing whole bananas  in the fridge is 2-3 days, but you are dealing with mashed fruit, which means a much larger surface area and some broken cell walls. So one or two days should be ok. But if you notice any signs of spoilage like discoloration apart from the aforementioned browning, visible mold or a strange smell (acidic, boozy, or just “off”), discard them regardless. The general rule is always: 


  When in doubt, throw it out.