Nicene and Ante-Nicene Fathers, Ser. II, Vol I: The Life of Constantine with Orations of Constantine and Eusebius.: Chapter XXI
Chapter XXI.—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, 3320 but only by the standard of the cross.
Compare for these, Yates, article Signa Militaria in Smith, Dict. Gr. and Rom. Ant., where there is given cut of the arch of Constantine showing such standards.