The quotation is from Aristotle's Peri psuches (De anima in Latin, On the Soul in English), book 1, chapter 4. (I apologize for not having the Bekker number for the passage, but book and chapter will make it easy to find in any translation.)

Here is the paragraph in which it occurs, J.A. Smith translation (University of Adelaide makes this available); I have capitalized the crucial sentence:

The case of mind is different; it seems to be an independent substance implanted within the soul and to be incapable of being destroyed. If it could be destroyed at all, it would be under the blunting influence of old age. What really happens in respect of mind in old age is, however, exactly parallel to what happens in the case of the sense organs; if the old man could recover the proper kind of eye, he would see just as well as the young man. The incapacity of old age is due to an affection not of the soul but of its vehicle, as occurs in drunkenness or disease. Thus it is that in old age the activity of mind or intellectual apprehension declines only through the decay of some other inward part; mind itself is impassible. Thinking, loving, and hating are affections not of mind, but of that which has mind, so far as it has it. That is why, when this vehicle decays, memory and love cease; they were activities not of mind, but of the composite which has perished; MIND IS, NO DOUBT, SOMETHING MORE DIVINE AND IMPASSIBLE. That the soul cannot be moved is therefore clear from what we have said, and if it cannot be moved at all, manifestly it cannot be moved by itself.

brief comment: nous can be rendered by "mind" or "intellect"; theioteron by "more divine" or "more godlike"; apathes by "unchanging" or "impassible."