"EVERY art (technê) and every inquiry (methodos), and similarly every action
(praxis) and pursuit (proairesis) , is thought to (dokei) aim at some good;
and for this reason the good has rightly been declared to be that at which
all things aim (ephiethai)." (Ross)

"the good has rightly been declared" by who? Obviously Aristotle is citing a
current opinion. Gauthier & Jolif point to a similar citation in Book 10 ch.
2, that gives us as possible source, Eudoxus, a hedonist, presumably of the
Philebus type (Aquinas does not make this connection):

"Eudoxus thought pleasure was the good because he saw all things, both
rational and irrational, aiming at it, and because in all things that which
is the object of choice is what is excellent, and that which is most the
object of choice the greatest good; thus the fact that all things moved
towards the same object indicated that this was for all things the chief
good (for each thing, he argued, finds its own good, as it finds its own
nourishment); and that which is good for all things and at which all aim was
the good."

So Aristotle is opening his Ethics by citing common views. The first, that
all actions and pursuits aim as some kind of a good (be it a selfish one) is
really common sense, and most ordinary Athenians could have easily agreed.
The second line, that picks out one single greatest good is already
philosophical.

Thomas