Cicero, On the Nature of the Gods 2.44 (Aristotle, On Philosophy, fr. 24 Rose 1886)

Aristotle, too, is to be praised for judging that all things that move are moved either by nature or by force or by [their own] will. The sun and moon and all the stars move; but things which are moved by nature are carried either downwards by their weight or upwards by their lightness, and neither of these applies to the stars, because their movement is round in a circle. Nor can it be said that some greater force brings it about that the stars move contrary to their nature; for what greater [force] can there be? So what is left is that the movement of the stars is voluntary.

Cicero, Academica Posteriora 1.26

Aristotle thought that there was a fifth kind [of element], from which were stars and minds; it was unique and unlike the four which I mentioned above.