Probably no Scriptural ref that most Protestants would accept.
The classic OT Catholic go to verse:
2 Mac 12:39-46
Next day, with Judas at their head, they went back to recover the bodies of the slain, for burial among their own folk in their fathers’ graves; 40 and what found they? Each of the fallen was wearing, under his shirt, some token carried away from the false gods of Jamnia. Here was defiance of the Jewish law, and none doubted it was the cause of their undoing; 41 none but praised the Lord for his just retribution, that had brought hidden things to light; 42 and so they fell to prayer, pleading that the sin might go unremembered.43 Then he would have contribution made; a sum of twelve thousand silver pieces he levied, and sent it to Jerusalem, to have sacrifice made there for the guilt of their dead companions. Was not this well done and piously? Here was a man kept the resurrection ever in mind; 44 he had done fondly and foolishly indeed, to pray for the dead, if these might rise no more, that once were fallen! 45 And these had made a godly end; could he doubt, a rich recompense awaited them? 46 A holy and wholesome thought it is to pray for the dead, for their guilt’s undoing.
Generally accepted by many older Churches as condolencse to a family, praise for the works of the departed and ending in a prayer for recently departed brother, a prayer for him to find mercy with the Lord ("when that day comes" meaning on Judgment day).
2 Tim 1:16-18
16 May the Lord grant mercy to the household of Onesiphorus;[3] often enough he revived my spirits. Instead of being ashamed of a prisoner’s acquaintance, 17 he sought me out when he was in Rome, and succeeded in finding me. 18 The Lord grant that he may find mercy with his Lord when that day comes; what he did for me in Ephesus I have no need to tell thee.
The rest from Scripture are less direct and most Protestors may scoff. Jesus making ref to forgiveness of sins mentions both "in this world" and the "in world to come". A prayer that the deceased find mercy is in essence and of necessity that they have forgiveness of sins. The fact Saint Paul could be seen as admonishing the manner but not the "thought"(like the OT passage above) of doing something (like pray) for the dead when the Saint spoke about "Baptism for the dead", this is at least evidence that people believed it possible to be able to do things that would benefit the dead.
The support from tradition is much stronger, beginning with the most ancient of Liturgies all having a prayer for the departed included one of few things that seems universal to all ancient Liturgies of the Mass - also something still found at every Mass conducted today. Some of those ancient versions included specific pleas, like Saint Paul's, for the remission of sins. Which further supports a very early and universal establishment of a practice in the whole Church everywhere without objection (doesn't happen a lot), which would also be odd absent appeal to Apostolic teaching on it, and that lends further credence to the traditional understanding of NT references mentioned.
