Constantine's Letter to Alexander the Bishop, and Arius the Presbyter.
Life of Constantine, with Orations · Eusebius of Caesarea
EusVC 3.123.1
“Victor Constantinus, Maximus Augustus, to Alexander and Arius.
EusVC 3.123.1
EusVC 3.123.2
“I call that God to witness, as well I may, who is the helper of my endeavors, and the Preserver of all men, that I had a twofold reason for undertaking that duty which I have now performed.