In De ente et essentia D. Thomae Aquinatis commentaria

He shows that essence is properly said of substances, but of accidents only in a qualified sense. The essences of simple substances are nobler than the essences of composite substances. The essence of a composite substance is neither matter nor form, nor the relation between them, nor finally anything added to the composite, but the composite itself.

But since being is said absolutely and first of substances, and later and in a qualified sense of accidents, it follows that essence is properly and truly in substances, but is in accidents in a certain way and in a qualified sense, and so forth.

Having completed the first of the proposed matters, he carries out in this second part of the treatise how essence is found in different things in different ways, which was proposed second. First he declares how essence is found in substances in three chapters; second, in separate substances in two chapters; third, in accidents in a single chapter. Concerning composite substances, three things are declared in those three chapters: in the first, what the essence of composite substances is; in the second, how the essences of composite substances differ from one another; in the third, how they are related to logical intentions. Beginning to carry out in this chapter how essence is found differently in different things, he first declares this in general, comparing all the essences of the universe with one another in two comparisons. Second, he explains what the essence of composite substances is through three negative conclusions and a fourth affirmative one. Third, he raises and solves a certain doubt concerning the affirmative conclusion. First he makes the first comparison between the essences of substances and accidents with this conclusion: Substance truly and properly has essence; accident has it in a qualified sense. He proves it as follows: each thing has essence in the same way as it is a being; but substance is a being without qualification, whereas accident is a being in a qualified sense; therefore substance has essence without qualification, whereas accident has it in a qualified sense. The major is evident from what was said in the preceding chapter. Although the minor is found at the beginning of VII Metaphysics, it is nevertheless proved in the text as follows: that is a being without qualification of which being is predicated absolutely and first, and that is a being in a qualified sense of which being is said later and in a qualified sense. But being is said absolutely and first of substance, and later and in a qualified sense of accident. Therefore substance is a being without qualification, whereas accident is a being in a qualified sense.