Library Eusebius of Caesarea Life of Constantine
Book 4, Chapter 14: How the Zealous Prayers of Constantine procured Peace to the Christians.
Thus, the nations of the world being everywhere guided in their course as it were by the skill of a single pilot, and acquiescing in the administration of him who governed as the servant of God, the peace of the Roman empire continued undisturbed, and all classes of his subjects enjoyed a life of tranquillity and repose. At the same time the emperor, who was convinced that the prayers of godly men contributed powerfully to the maintenance of the public welfare, felt himself constrained zealously to seek such prayers and not only himself implored the help and favor of God, but charged the prelates of the churches to offer supplications on his behalf.
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.