From Book I, Chapter I of his De fide:
Now this is the declaration of our Faith, that we say that God is One, neither dividing His Son from Him, as do the heathen, nor denying, with the Jews, that He was begotten of the Father before all worlds, and afterwards born of the Virgin; nor yet, like Sabellius, confounding the Father with the Word, and so maintaining that Father and Son are one and the same Person; nor again, as does Photinus, holding that the Son first came into existence in the Virgin's womb: nor believing, with Arius, in a number of diverse Powers, and so, like the benighted heathen, making out more than one God. For it is written: "Hear, O Israel: the Lord your God is one God."
For God and Lord is a name of majesty, a name of power, even as God Himself says: "The Lord is My name," and as in another place the prophet declares: "The Lord Almighty is His name." God is He, therefore, and Lord, either because His rule is over all, or because He beholds all things, and is feared by all, without difference.
If, then, God is One, one is the name, one is the power, of the Trinity. Christ Himself, indeed, says: "Go, baptize the nations in the name of the Father, and of the Son, and of the Holy Spirit." In the name, mark you, not in the names.