Thomas Aquinas, Compendium of Theology

It is also clear that there can be no accident in God. If all perfections are one in Him, and if existence, power, action, and all such attributes pertain to perfection, they are necessarily all identical with His essence. Therefore none of these perfections is an accident in God.

Furthermore, a being to whose perfection something can be added, cannot be infinite in perfection. But if a being has some perfection that is an accident, a perfection can be added to its essence, since every accident is superadded to essence. Hence infinite perfection will not be found in its essence. But, as we have shown, God is of infinite perfection according to essence. Consequently there can be in Him no accidental perfection; whatever is in Him, is His substance.

The same truth can be easily inferred from God's supreme simplicity, and from the fact that He is pure act and is the first among beings. For some sort of composition obtains between an accident and its subject. Likewise, that which is a subject of this kind cannot be pure act, since an accident is a certain form or act of the subject. Similarly, what is per se always precedes what is per accidens. From all this we can infer, in keeping with the truths established above, that nothing can be predicated of God as an accident.