The Complete Aristotle (eng.)

magnitude. Locomotion, it is true, we cannot
show to be finite in this way, since it is not always between
contraries. But since that which cannot be cut (in the sense that
it is inconceivable that it should be cut, the term ‘cannot’ being
used in several senses)-since it is inconceivable that that which
in this sense cannot be cut should be in process of being cut, and
generally that that which cannot come to be should be in process of
coming to be, it follows that it is inconceivable that that which
cannot complete a change should be in process of changing to that
to which it cannot complete a change. If, then, it is to be assumed
that that which is in locomotion is in process of changing, it must
be capable of completing the change. Consequently its motion is not
infinite, and it will not be in locomotion over an infinite
distance, for it cannot traverse such a distance.
    It is evident, then, that a process of change cannot be infinite
in the sense that it is not defined by limits. But it remains to be
considered whether it is possible in the sense that one and the
same process of change may be infinite in respect of the time which
it occupies. If it is not one process, it would seem that there is
nothing to prevent its being infinite in this sense; e.g. if a
process of locomotion be succeeded by a process of alteration and
that by a process of increase and that again by a process of coming
to be: in this way there may be motion for ever so far as the time
is concerned, but it will not be one motion, because all these
motions do not compose one. If it is to be one process, no motion
can be infinite in respect of the time that it occupies, with the
single exception of rotatory locomotion.

Physics, Book VII
    Translated by R. P. Hardie and R. K. Gaye
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1
    Everything that is in motion must be moved by something. For if
it has not the source of its motion in itself it is evident that it
is moved by something other than itself, for there must be
something else that moves it. If on the other hand it has the
source of its motion in itself, let AB be taken to represent that
which is in motion essentially of itself and not in virtue of the
fact that something belonging to it is in motion. Now in the first
place to assume that AB, because it is in motion as a whole and is
not moved by anything external to itself, is therefore moved by
itself-this is just as if, supposing that KL is moving LM and is
also itself in motion, we were to deny that KM is moved by anything
on the ground that it is not evident which is the part that is
moving it and which the part that is moved. In the second place
that which is in motion without being moved by anything does not
necessarily cease from its motion because something else is at
rest, but a thing must be moved by something if the fact of
something else having ceased from its motion causes it to be at
rest. Thus, if this is accepted, everything that is in motion must
be moved by something. For AB, which has been taken to represent
that which is in motion, must be divisible since everything that is
in motion is divisible. Let it be divided, then, at G. Now if GB is
not in motion, then AB will not be in motion: for if it is, it is
clear that AG would be in motion while BG is at rest, and thus AB
cannot be in motion essentially and primarily. But ex hypothesi AB
is in motion essentially and primarily. Therefore if GB is not in
motion AB will be at rest. But we have agreed that that which is at
rest if something else is not in motion must be moved by something.
Consequently, everything that is in motion must be moved by
something: for that which is in motion will always be divisible,
and if a part of it is not in motion the whole must be at rest.
    Since everything that is in motion must be moved by something,
let us take the case in which a thing is in locomotion and is moved
by something that is itself in motion, and that again is moved by
something else that is in motion, and that by something else, and
so on continually: then the series cannot go on to infinity, but
there must be some first movent. For let us suppose that this is
not so and take the series to be infinite. Let A then be moved by
B, B by G, G by D, and so on, each member of the series being moved
by that which comes next to it. Then since ex hypothesi the movent
while causing motion is also itself in motion, and the motion of
the moved and the motion of the movent must