Library Eusebius of Caesarea Church History
Book 7, Chapter 6: The Heresy of Sabellius.
He refers also in the same letter to the heretical teachings of Sabellius, which were in his time becoming prominent, and says:
For concerning the doctrine now agitated in Ptolemais of Pentapolis, — which is impious and marked by great blasphemy against the Almighty God, the Father, and our Lord Jesus Christ, and contains much unbelief respecting his Only Begotten Son and the first-born of every creature, the Word which became man, and a want of perception of the Holy Spirit — as there came to me communications from both sides and brethren discussing the matter, I wrote certain letters treating the subject as instructively as, by the help. of God, I was able. Of these I send you copies.
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.