Library Eusebius of Caesarea Life of Constantine
Book 3, Chapter 24: How he wrote Frequent Letters of a Religious Character to the Bishops and People.
But besides these, his writings are very numerous on kindred subjects, and he was the author of a multitude of letters, some to the bishops, in which he laid injunctions on them tending to the advantage of the churches of God; and sometimes the thrice blessed one addressed the people of the churches generally, calling them his own brethren and fellow-servants. But perhaps we may hereafter find leisure to collect these dispatches in a separate form, in order that the integrity of our present history may not be impaired by their insertion.
English translation by Ernest Cushing Richardson, 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.