Church History by Eusebius Pamphilius
Chapter XXVIII.--The Persecution under Maximinus.
The Roman emperor, Alexander, having finished his reign in thirteen years, was succeeded by Maximinus Cæsar. On account of his hatred toward the household of Alexander, which contained many believers, he began a persecution, commanding that only the rulers of the churches should be put to death, as responsible for the Gospel teaching. Thereupon Origen composed his work On Martyrdom, and dedicated it to Ambrose and Protoctetus, a presbyter of the parish of Cæsarea, because in the persecution there had come upon them both unusual hardships, in which it is reported that they were eminent in confession during the reign of Maximinus, which lasted but three years. Origen has noted this as the time of the persecution in the twenty-second book of his Commentaries on John, and in several epistles.