Library Eusebius of Caesarea Life of Constantine
Book 1, Chapter 48: Celebration of Constantine's Decennalia.
While he was thus circumstanced, he completed the tenth year of his reign. On this occasion he ordered the celebration of general festivals, and offered prayers of thanksgiving to God, the King of all, as sacrifices without flame or smoke. And from this employment he derived much pleasure: not so from the tidings he received of the ravages committed in the Eastern provinces.
English translation by Ernest Cushing Richardson, in Nicene and Post-Nicene Fathers, Series II, Vol. I (eds. Philip Schaff and Henry Wace; Buffalo: Christian Literature Publishing Co., 1890). Public domain. Digital text via New Advent Church Fathers.