We should note that in every genus the cause is what is first in that genus, by which all the things that belong to that genus are constituted in that genus, such as among the elementary bodies, fire is the first hot thing, from which all things obtain heat. Now, one does not proceed into infinity in any order of things. So in the order of beings something must be first, which gives being to all.
Aquinas, Commentary on the Book of Causes, proposition 18; Guagliardo, tr. (CUA Press, 1996) p. 113. He also goes on (following the author of the Liber de Causis) to argue for the same thing in the order of life and the order of knowledge, thus giving the three grades of nobility: being, living, and knowing. (A very interesting feature of Aquinas's discussion in this context is that living makes the list because it is self-motion, which is the noblest form of tending or mobility; and knowing makes the list because it is purely formal possession, which is the noblest form of having or possession. Being, tending, having.) This gives the explanation for a fact that often puzzles people, i.e., the fact that Aquinas explicitly mentions nobility in the Fourth Way.
The Latin:
Considerandum est quod in unoquoque genere est causa illud quod est primum in genere illo, a quo omnia quae sunt illius generis in illo genere constituuntur, sicut inter elementaria corpora ignis est primum calidum a quo omnia caliditatem sortiuntur; non est autem in aliquo rerum ordine in infinitum procedere. Oportet igitur in ordine entium esse aliquod primum quod dat omnibus esse, et hoc est quod dicit quod res omnes habent essentiam per ens primum.