Handbook on Faith, Hope, and Charity 18:69 [A.D. 421]
"We read in the books of the Maccabees [2 Macc. 12:43] that sacrifice was offered for the dead. But even if it were found nowhere in the Old Testament writings, the authority of the Catholic Church which is clear on this point is of no small weight, where in the prayers of the priest poured forth to the Lord God at his altar the commendation of the dead has its place."
The Care to be Had for the Dead 1:3 [A.D. 421]