The simplest answer could be either the exercise wheel or the cage itself (if the bottom is wire). Try taking the wheel out for a day or two and see if there's improvement. Make sure the bedding of the cage is suitable to cover the whole bottom as well. (Are your hamster's nails being trimmed properly, or kept to a healthy length by other means?)
What type of hamster is this?
It could also be the start of a bacterial infection, or something more serious. I'd take him to the vet if you try the first options and don't see any improvement, or worse, the condition spreads.
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