Adam Clarke on Philemon 1:23

Epaphras, my fellow prisoner - Epaphras was a Colossian, as we learn from Col 4:12 : Epaphras, who is one of you. But there is no account there of his being in prison, though the not mentioning of it does not necessarily imply that he was not. Some time or other he had suffered imprisonment for the truth of the Gospel; and on that account St. Paul might, in a general way, call him his fellow prisoner.