New Advent, 2 Maccabees 12: 40—ff.
2 Maccabees (it is relegated to the Apocrypha in Protestant Bibles that have it at all):
12:40. And they found under the coats of the slain, some of
Of the donaries, etc... That is, of the votive offerings, which had been hung up in the temples of the idols, which they had taken away when they burnt the port of Jamnia, verse 9, contrary to the prohibition of the law, Deuteronomy 7:25.
12:41. Then they all blessed the just judgment of the Lord, who had discovered the things that were hidden.
[Omnes itaque benedixerunt iustum iudicium Domini qui occulta fecerit manifesta]
12:42. And so betaking themselves to prayers, they besought him, that the sin which had been committed might be forgotten. But the most valiant Judas exhorted the people to keep themselves from sin, forasmuch as they saw before their eyes what had happened, because of the sins of those that were slain.
[Atque ita ad preces conversi rogaverunt ut id quod factum erat delictum obliteraretur at vero fortissimus Iudas hortabatur populum conservare se sine peccato sub oculis videntes quae facta sint pro peccato eorum qui prostrati sunt]
12:43. And making a gathering, he sent twelve thousand drachms of silver to Jerusalem for sacrifice to be offered for the sins of the dead, thinking well and religiously concerning the resurrection.
[Et facta conlatione duodecim milia dragmas argenti misit Hierosolymam offerri pro peccato sacrificium bene et religiose de resurrectione cogitans]
12:44. (For if he had not hoped that they that were slain should rise again, it would have seemed superfluous and vain to pray for the dead),
[Nisi enim eos qui ceciderant resurrecturos speraret superfluum videretur et vanum orare pro mortuis]
12:45. And because he considered that they who had fallen asleep with godliness, had great grace laid up for them.
[Et quia considerabat quod hii qui cum pietate dormitionem acceperant optimam haberent repositam gratiam]
With godliness... Judas hoped that these men who died fighting for the cause of God and religion, might find mercy: either because they might be excused from mortal sin by ignorance; or might have repented of their sin, at least at their death.
12:46. It is therefore a holy and wholesome thought to pray for the dead, that they may be loosed from sins.
[Sancta ergo et salubris cogitatio pro defunctis exorare ut a peccato solverentur]
It is therefore a holy and wholesome thought to pray for the dead... Here is an evident and undeniable proof of the practice of praying for the dead under the old law, which was then strictly observed by the Jews, and consequently could not be introduced at that time by Judas, their chief and high priest, if it had not been always their custom.
I’ve recently read that the first Nov. 1st celebrations for All Saints Day occurred in the middle of the 2nd century, prompted by the death of St. Polycarp, a student of St. John the Evangelist. That’s been stated in many blogs, but I couldn’t find an authoritative source.