Library Eusebius of Caesarea Church History
Book 6, Chapter 35: Dionysius succeeds Heraclas in the Episcopate.
In the third year of this emperor, Heraclas died, having held his office for sixteen years, and Dionysius received the episcopate of the churches of Alexandria.
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.