Text #6201A select library of Nicene and Post-Nicene Fathers of the Christian Church, edited by , Philip & , Henry , "Life of Constantine the Great", in
[Euseb. Const. 1.28. The Christian Literature Company. 1890 p. 490]
How, while he was praying, God sent him a Vision of a Cross of Light in the Heavens at Mid-day, with an Inscription admonishing him to conquer by that.
Accordingly he called on him with earnest prayer and supplications that he would reveal to him who he was, and stretch forth his right hand to help him in his present difficulties. And while he was thus praying with fervent entreaty, a most marvelous sign appeared to him from heaven, the account of which it might have been hard to believe had it been related by any other person. […] He said that about noon, when the day was already beginning to decline, he saw with his own eyes the trophy of a cross of light in the heavens, above the sun, and bearing the inscription, CONQUER BY THIS. At this sight he himself was struck with amazement, and his whole army also, which followed him on this expedition, and witnessed the miracle.