There's certainly more to be said, but I commented on a very similar question a couple years ago. The earliest call for abolition I've ever found is from Bishop Gregory of Nyssa in the 4th Century CE.

The one thing that I wish I could add to the answer in the link is that, even if Gregory is the oldest surviving abolitionist, there's no proof one way or the other to suggest that he was the first abolitionist ever. Most of the writing from antiquity is lost, and most of the people didn't write anything in the first place. However, the resounding silence from all of our sources does make it pretty clear that there was never an ancient abolition movement. For most of recorded history it seems the best you'd find were individuals with abolitionist ideas.