Alexander of Alexandria: Epistle to Alexander of Byzantium - 324 A.D.

"..At this same period also, Alexander, illustrious for his apostolical gifts, governed the church of Constantinople
It was at this time that Alexander, bishop of Alexandria, perceiving that Arius, enslaved by the lust of power, was assembling those who had been taken captive by his blasphemous doctrines, and was holding private meetings, communicated an account of his heresy by letter to the rulers of the principal churches. That the authenticity of my history may not be suspected, I shall now insert in my narrative the letter which he wrote to his namesake.. "

The Ecclesiastical History, Dialogues, and Letters of Theodoret.