It is in the Bible. In a book that Luther does not wish to include as God inspired Scriptures.
2nd Maccabees Chapter 12:39 On the following day, since the task had now become urgent, Judas and his companions went to gather up the bodies of the fallen and bury them with their kindred in their ancestral tombs.
40 But under the tunic of each of the dead they found amulets sacred to the idols of Jamnia, which the law forbids the Jews to wear. So it was clear to all that this was why these men had fallen.
41 They all therefore praised the ways of the Lord, the just judge who brings to light the things that are hidden.
42 Turning to supplication, they prayed that the sinful deed might be fully blotted out. The noble Judas exhorted the people to keep themselves free from sin, for they had seen with their own eyes what had happened because of the sin of those who had fallen.
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43 He then took up a collection among all his soldiers, amounting to two thousand silver drachmas, which he sent to Jerusalem to provide for an expiatory sacrifice. In doing this he acted in a very excellent and noble way, inasmuch as he had the resurrection in mind;
44 for if he were not expecting the fallen to rise again, it would have been superfluous and foolish to pray for the dead.
45 But if he did this with a view to the splendid reward that awaits those who had gone to rest in godliness, it was a holy and pious thought.
46 Thus he made atonement for the dead that they might be absolved from their sin.
Remember that it is only in the Book of Maccabees you can find the story of Hanukkah which is celebrated by Jews during December.
Agree and this is why Judas Maccabees pray for the dead.