Church History by Eusebius Pamphilius
Chapter XX.--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.