Peter Hunt writes in his essay "Slaves in Greek Literary Culture" found in The Cambridge History of World Slavery Vol. 1, there is a bit of anti-slavery writing going as far back as ancient Greece. Hunt claims that "a number of scholars have" regarded Euripides "in particular, as a proto- or a crypto-abolitionist". Among other evidence in Euripedes' writing, he wrote in a passage in Hecuba:

"Alas, what an evil slavery has always been

"It endures what is not right,

"overcome by force."

Or in another translation:

"Alas! how cursed is slavery always in its nature / forced by the might of the stronger / to endure unseemly treatment."

Antisthenes, the reported son of a Thracian slave woman, wrote an entire treatise On Freedom and Slavery, which is lost, but which Hunt suspects may have been an anti-slavery text, based on the one quote from the original work that survives.

Historian Peter Garnsey argues in Ideas of Slavery from Aristotle to Augustine that there was "rather more than a universal, passive acceptance of the institution" of slavery, and criticism is found throughout ancient texts, albeit usually in rather modest terms in the texts that do survive. That's probably because, as Hunt writes:

"[The] mass of evidence is, however, obviously one-sided. The surviving literature of classical Greece was almost all written by slave masters and for its free, male citizens."

Garnsey argues that, despite the lack of anti-slavery texts, due to the dominance of slaveholders as the writers of the surviving texts, it can nonetheless be inferred that there was vocal anti-slavery rhetoric at the time. After all, if Aristotle is giving a defense of natural slavery, what is he defending it against if there isn't criticism of it? Garnsey writes:

"At one point in the Politics, Aristotle refers to the view ascribed to certain unnamed persons that slavery was unjust, inasmuch as it was a product of convention and rested on nothing else than superior force. Aristotle's own conviction was that some people are natural free men while others are natural slaves.

"Here are the beginnings of a critique of slavery on moral grounds. Aristotle gives few clues as to who advanced it. They are usually regarded as a few isolated individuals. Rather more than this is suggested by 'many of those versed in the law' and 'some men of judgement' (sophoi). They were perhaps 'philosophers of law'- not, in any case, jurists, there having been no such profession in Athens. The intellectual origins of the critique lie ultimately in the activity of the sophists, which was centred in the second half of the fifth century and the early fourth and was marked by sceptical inquiry into traditional beliefs and practices. How far the critique was taken is a moot point, but Aristotle evidently thought that it was sufficiently dangerous to warrant a counter-attack."

However, Hunt counters that these kinds of arguments might be overstating the case from the available surviving texts. The texts of Euripides, and other suggested critics such as Phormio and Pasion, fall "well short" of being abolitionist. Rather than being critical to the point of advocating for abolition, the surviving texts are more suggestive, says Hunt, of the viewpoint that slavery was considered "inevitable and necessary" in ancient Greece, and that slaves were to be pitied, rather than that abolition was to be championed.

But again, as Hunt says, all we really have are the writings of slaveholders. We have no direct evidence of what the non-slaveholding class of free Greek citizens (let alone of what the underclasses) thought of slavery. Thus, unlike Garnsey who goes further, Hunt concludes it's "possible but not certain" that there was criticism of slavery at the time, and that there were abolitionists, but the biased evidence is too scant to make any solid conclusions.

Similarly, Neville Morley writes in "Slavery Under The Principate" also published in The Cambridge World History of Slavery Vol. 1, that in ancient Rome "there is no evidence for any ancient abolitionist movement, let alone for its having any effect on social practices". Catherine Hezser writes in her essay "Slavery and the Jews": "The claim that ancient Jewish sources reveal a proto-abolitionist stance towards slavery can therefore be considered apologetic rather than historically persuasive."

The first real, unequivocal anti-slavery text dating from antiquity is probably Gregory of Nyssa's "Fourth Homily on Ecclesiastes", written in the 4th century C.E. Only part of the sermon survives, the most often quoted passage being:

"I got me slaves and slave-girls. For what price, tell me? What did you find in existence worth as much as this human nature? What price did you put on rationality? How many obols did you reckon the equivalent of the likeness of God? How many staters did you get for selling the being shaped by God? God said, let us make man in our own image and likeness. If he is in the likeness of God, and rules the whole earth, and has been granted authority over everything on earth from God, who is his buyer, tell me? Who is his seller? To God alone belongs this power; or rather, not even to God himself. For his gracious gifts, it says, are irrevocable. God would not therefore reduce the human race to slavery, since he himself, when we had been enslaved to sin, spontaneously recalled us to freedom. But if God does not enslave what is free,who is he that sets his own power above God’s?"

Jennifer Glancy writes of Gregory's text in "Slavery And The Rise Of Christianity" published in the Cambridge World History of Slavery:

"In his fourth homily on Ecclesiastes, Gregory of Nyssa penned what may be the most scathing ancient Christian critique of slavery, a critique that, though not setting forth a programme of reform, anticipates some of the hermeneutical moves of nineteenth-century Christian abolitionists...The brunt of Gregory’s attack was slaveholder arrogance; he did not explicitly call for the abolition of slavery or even wholesale manumission of slaves. However, his condemnation of the morality of slaveholding comes as close as anything in extant early Christian literature to a sustained attack on the foundations of slavery."

D. Bentley Hart writes in "The ‘Whole Humanity’ Gregory of Nyssa's Critique of Slavery in Light of His Eschatology" published in the Scottish Journal of Theology:

"Nowhere in the literary remains of antiquity is there another document quite comparable to Gregory of Nyssa's fourth homily on the book of Ecclesiastes: certainly no other ancient text still known to us—Christian, Jewish, or Pagan—contains so fierce, unequivocal, and indignant a condemnation of the institution of slavery....[I]t is a passage of remarkable rhetorical intensity. In it Gregory treats slavery not as a luxury that should be indulged in only temperately (as might an Epicurean), nor as a necessary domestic economy too often abused by arrogant or brutal slave-owners (as might a Stoic like Seneca or a Christian like John Chrysostom), but as intrinsically sinful, opposed to God's actions in creation, salvation, and the church, and essentially incompatible with the Gospel."

From then on, evidence of anti-slavery began appearing. As some examples, in 873 CE, shortly after becoming pope, John VIII wrote to the princes of Sardinia commanding them to free all their slaves in order to save their souls and repent from sin. As recorded in the Sacrorum Conciliorum Nova et Amplissima Collectio Vol. XVIIIa, at the Council of Koblenz in 922 CE, the Holy Roman Empire tried to enact a law that considered the enslavement of Christians as illegal:

"Also the question was put what should be done concerning him who led away a Christian man and then sold him; and the reply of all was that he should be guilty of homicide."

At the 1102 Council of London, the slave trade was banned as "wicked":

"Let no man from henceforth presume to carry on that wicked traffic, by which men in England have been hitherto sold like brute animals."

The movement away from slavery in Europe continued through the centuries. By the time the African slave trade began c.1500, much of Europe was free. But then, of course, with the colonization of the Americas, it was instituted against African captives. This began a whole new round of anti-slavery rhetoric, among the earliest being those written by the missionary Bartolomé de las Casas in A Short Account of the Destruction of the Indies. This helped enact abolition of enslavement of the indigenous people of the Americas through the New Laws of 1542, in the Holy Roman Empire.

Unfortunately, in de las Casas's book, he suggested the enslavement of Africans as a replacement of the enslavement of the indigenous peoples, a suggestion he quickly came to regret. Writing in the second volume of his three-volume History of the Indies:

"I soon repented and judged myself guilty of ignorance. I came to realize that black slavery was as unjust as Indian slavery... and I was not sure that my ignorance and good faith would secure me in the eyes of God."

He wouldn't be the last abolitionist and critic of slavery. On the contrary, throughout the period of the African slave trade in the Americas, there was a corresponding anti-slavery movement. Wherever slavery spread, anti-slavery texts soon followed.

TL;DR: There is indirect evidence of anti-slavery thought in the ancient world, but since surviving texts are almost all written by slaveholders, the inferences are debated by scholars. It isn't until the 4th century C.E. that an unequivocal anti-slavery text is preserved.