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.
Historia Ecclesiastica
Book VI, Chapter 35. Dionysius succeeds Heraclas in the Episcopate
1 section
Eusebius of Caesarea
Trans. Arthur Cushman McGiffert, A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Volume I, 1890. Public domain.