Library Eusebius of Caesarea Church History
Book 6, Chapter 26: Heraclas becomes Bishop of Alexandria.
It was in the tenth year of the above-mentioned reign that Origen removed from Alexandria to Cæsarea, leaving the charge of the catechetical school in that city to Heraclas. Not long afterward Demetrius, bishop of the church of Alexandria, died, having held the office for forty-three full years, and Heraclas succeeded him. At this time Firmilianus, bishop of Cæsarea in Cappadocia, was conspicuous.
English translation by Arthur Cushman McGiffert, 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.