Library Eusebius of Caesarea Life of Constantine
Book 4
Chapter 21: He orders the Sign of the Saviour's Cross to be engraven on his Soldiers' Shields
And not only so, but he also caused the sign of the salutary trophy to be impressed on the very shields of his soldiers; and commanded that his embattled forces should be preceded in their march, not by golden images, as heretofore, but only by the standard of the cross.
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.