The idea that dark people are cowards, and light people courageous fighters, is found already in Airs, Waters, Places.
Airs, Waters, Places is a treatise by Hippokrates of Kos that deals with the effects of the physical environment on living organisms over an extended period of time, and the exact quote Isaac has in mind is:
And with regard to the pusillanimity and cowardice of the inhabitants, the principal reason the Asiatics are more unwarlike and of gentler disposition than the Europeans is, the nature of the seasons, which do not undergo any great changes either to heat or cold, or the like; for there is neither excitement of the understanding nor any strong change of the body whereby the temper might be ruffled and they be roused to inconsiderate emotion and passion, rather than living as they do always in the state. It is changes of all kinds which arouse understanding of mankind, and do not allow them to get into a torpid condition. For these reasons, it appears to me, the Asiatic race is feeble, and further, owing to their laws; for monarchy prevails in the greater part of Asia, and where men are not their own masters nor independent, but are the slaves of others, it is not a matter of consideration with them how they may acquire military discipline, but how they may seem not to be warlike, for the dangers are not equally shared, since they must serve as soldiers, perhaps endure fatigue, and die for their masters, far from their children, their wives, and other friends; and whatever noble and manly actions they may perform lead only to the aggrandizement of their masters, whilst the fruits which they reap are dangers and death; and, in addition to all this, the lands of such persons must be laid waste by the enemy and want of culture. Thus, then, if any one be naturally warlike and courageous, his disposition will be changed by the institutions. As a strong proof of all this, such Greeks or barbarians in Asia as are not under a despotic form of government, but are independent, and enjoy the fruits of their own labors, are of all others the most warlike; for these encounter dangers on their own account, bear the prizes of their own valor, and in like manner endure the punishment of their own cowardice. And you will find the Asiatics differing from one another, for some are better and others more dastardly; of these differences, as I stated before, the changes of the seasons are the cause. Thus it is with Asia.
Source: On Airs, Waters, and Places, by Hippocrates. Translated by Francis Adams, provided by The Internet Classics Archive.
I've found several other sources that discuss scientific racism in ancient Greece, however they either revolve around the aforementioned quote, or don't provide any references at all. Are there any other prominent examples of scientific racism in ancient Greece?
I'd appreciate answers with references to scientific writings of the time, and if possible by authors we know were influential during their lifetime (e.g. Aristotle, Plato, etc).