There is no single 'labor theory of value'; labor theories of value can be quite different depending on what aspect of labor they consider relevant. But labor theories in general will hold that in some sense the price of something is equivalent (by some measure) to the work required to have it (by some measure). More precise versions distinguish value in use and value in exchange; in Adam Smith's famous example, few things have greater value in use than water, which is one of the most useful and one of the most necessary things we know, but water has in most situations almost no value in exchange; it's not particularly useful for buying and selling. The conclusion can then be drawn that the correct measurement for value in exchange is quantity of labor (whatever the measure of quantity might be). It is a common view that the labor theory of value goes back to Aristotle, and that it is a standard position in Aristotelianism, but this is not, in fact, true. A few gestures toward the reason why.
(1) Aristotle does not have a labor theory of value because he has no specific theory of value of the relevant sort. He does have a sketched-out theory of commercial exchange; this theory recognizes that exchanges originally are based on mutually recognized value in use. Because direct exchange is often not practical (due to things like transportation, storage, timing), we move from direct exchange of particular useful thing for particular useful thing to indirect exchange, where we exchange indirectly using something that has general usefulness. Metals are durable and relatively easy to transport and exhange, and even if you yourself have no particular use for it, lots of other people do, so you can use it in further exchanges. This indirect exchange requires all sorts of measurements, protections, and guarantees, and out of this comes our system of using money. With the advent of coinage as a standardized form of something generally useful that is specifically devoted to serving as something generally useful in exchange, we begin to get the notion of money-accumulation. In this context we have household management (Aristotle's word will later give us the word 'economics', which literally means household management; Aristotle himself would regard what we call 'economics' as politics), if we keep money-making tied to real usefulness, and money-exchange, if we treat the two as separate.
(2) It follows from this that the labor theory of value does not actually make much sense in an Aristotelian context; value is established by ends, and so will vary as ends vary. Aristotle's criticism of money-exchange has no direct relation to questions of labor; rather, the criticism is that good money-making makes money for definite ends, but money-exchange does not. Money-exchange treats money as an end in itself, and thus is unnatural. The only real connection to labor to which Aristotle appeals is that good money-making is something that he sees arising not out of labor as such but out of skill. I suppose you could argue on this basis that Aristotle has a skill theory of value, but even this would not really be accurate -- skill comes up simply because skills are a central part of human life that by their very nature involve ends.
(3) Marx often comes up in attributing the labor theory of value to Aristotle, because Marx builds his own labor theory of value on Aristotle. But I think close attention to the relevant texts shows that Marx himself did not think Aristotle had a labor theory of value; he thinks Aristotle laid some of the groundwork for it, but that he failed to solve a particular problem, how to have equal exchange of unlike things, and Marx proposes the labor theory of value as a solution to this problem that Aristotle did not solve -- in effect, we are actually exchanging A and B in terms of their labor, so the exchange is made in terms of something that can be directly compared on each side. If I read Marx correctly, I think he takes it to be the case that Aristotle couldn't have had a labor theory of value because the existence of slavery in Greek society would have made arguing for it at best very complicated and perhaps impossible. And if this is so, I think he is right here -- Aristotle can't really say that labor admits of equal comparison by quantity because he thinks some labor is better than other labor by its very nature. Some labor is slavish, some labor is noble; some labor is suitable for a free person, some labor is not. In Marx's history of economics, the labor theory of value is the child of capitalism.
(4) If you wanted to find an Aristotelian labor theory of value, St. Thomas would probably be a better candidate than Aristotle himself. St. Thomas does explicitly at times link the value of something in exchange to labor. He also in a number of places uses examples explicitly based on labor -- for instance, he explains the proportional equality of justice in wages in terms of paying twice as much for twice as much time spent in labor. But St. Thomas is not a labor theorist, either, because he also thinks that share in the distribution of benefits is a matter of justice in exchange, and explicitly thinks that workmen should receive according to the quantity and quality of what they produce (which is not the same as the quantity of labor itself). And his account of why labor is relevant is that we price things according to the need we have to use them -- the farmer wouldn't trade a bushel of wheat for a sandal in general because the amount of labor that goes into a bushel of wheat is immense in comparison to that which goes into a pair of shoes, and if he were always laboring greatly to exchange very important things (like food, which is needed to live) for things that can be produced comparatively easily and are less important, he would not be meeting his needs in a reasonable way.
