Library Eusebius of Caesarea Life of Constantine
Book 4, Chapter 34: That he wrote to Eusebius respecting Easter, and respecting Copies of the Holy Scriptures.
Ever careful for the welfare of the churches of God, the emperor addressed me personally in a letter on the means of providing copies of the inspired oracles, and also on the subject of the most holy feast of Easter. For I had myself dedicated to him an exposition of the mystical import of that feast; and the manner in which he honored me with a reply may be understood by any one who reads the following letter.
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.