Library Eusebius of Caesarea Church History
Book 7, Chapter 20: The Festal Epistles of Dionysius, in which he also gives a Paschal Canon.
Dionysius, besides his epistles already mentioned, wrote at that time also his extant Festal Epistles, in which he uses words of panegyric respecting the passover feast. He addressed one of these to Flavius, and another to Domitius and Didymus, in which he sets forth a canon of eight years, maintaining that it is not proper to observe the paschal feast until after the vernal equinox. Besides these he sent another epistle to his fellow presbyters in Alexandria, as well as various others to different persons while the persecution was still prevailing.
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.