Library Eusebius of Caesarea Life of Constantine
Book 4, Chapter 58: Concerning the Building of a Church in Honor of the Apostles at Constantinople.
After this he proceeded to erect a church in memory of the apostles, in the city which bears his name. This building he carried to a vast height, and brilliantly decorated by encasing it from the foundation to the roof with marble slabs of various colors. He also formed the inner roof of finely fretted work, and overlaid it throughout with gold. The external covering, which protected the building from the rain, was of brass instead of tiles; and this too was splendidly and profusely adorned with gold, and reflected the sun's rays with a brilliancy which dazzled the distant beholder. The dome was entirely encompassed by a finely carved tracery, wrought in brass and gold.
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.