Philemon 1:13

‘Whom I would wish to have kept with me, that on your behalf he might minister to me in the bonds of the gospel,’

For Paul had wished to keep Onesimus with him, so that on behalf of Philemon and all who loved him, Onesimus might serve him in his imprisoned state, even in his bonds, which were for the sake of the Gospel (the bonds of the Gospel). This would have been of great benefit to him, but he refused to countenance his own comfort, if it meant compromising on what was right.