Formulated in precisely this way (namely, that God’s essence is identical to His attributes), probably the best source is St. Thomas Aquinas.
For example, in his Summa theologiae, Ia, q. 3, a. 6, in which he asks the question, “Does God have any accidents?” Aquinas answers in the negative, and the first objection regards the Divine Attributes:
[W]isdom, virtue, and the like, which are accidents in us, are attributes of God. Therefore in God there are accidents. [Note that this is an objection that Aquinas later refutes.]
Aquinas explains in the responsum that God cannot have any accidents for a variety of reasons: first, because the substance-accident composition is an actuality-and-potentiality pair which is impossible in God (who is the First Cause and thus Pure Act); second, because God’s Essence is identical with His Being, and hence unfettered and unlimited, which prevents anything extraneous from being “added” to it; finally, because God cannot not have “properties” that are distinct from Himself, since these would be “part” of Him and yet caused by Him, but there is nothing caused in God.
In response to the objection, he simply observes that wisdom, virtue, and similar attributes are not predicated of God and of man in exactly the same way. Although such attributes would be accidents in man, they are not so in God.
It follows that these attributes are identical with God. (We see this idea confirmed in other parts of the Summa, for example, Ia, q. 13, a. 4.)
This idea, however, was expressed earlier by the Fathers of the Church. For example, St. Augustine affirms it in his De Trinitate, V, 2:
But other things that are called essences or substances admit of accidents, whereby a change, whether great or small, is produced in them. But there can be no accident of this kind in respect to God; and therefore He who is God is the only unchangeable substance or essence, to whom certainly being itself, whence comes the name of essence, most especially and most truly belongs.
However, this is not Augustine’s invention: it was the universal consensus of the Fathers, including Clement of Alexandria, Basil, Cyril of Alexandria, John Damascene, and Pseudo-Dionysius the Areopagite.