Library Eusebius of Caesarea Life of Constantine
Book 2, Chapter 64: Constantine's Letter to Alexander the Bishop, and Ariusthe Presbyter.
Victor Constantinus, Maximus Augustus, to Alexander and Arius.
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.
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.