Library Eusebius of Caesarea Life of Constantine
Book 2, Chapter 7: That Victory everywhere followed the Presence of the Standard of the Cross in Battle.
Indeed, wherever this appeared, the enemy soon fled before his victorious troops. And the emperor perceiving this, whenever he saw any part of his forces hard pressed, gave orders that the salutary trophy should be moved in that direction, like some triumphant charm against disasters: at which the combatants were divinely inspired, as it were, with fresh strength and courage, and immediate victory was the result.
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.