From Plutarch:
The Achaeans voted Titus many honours, none of which seemed commensurate with his benefactions except one gift, and this caused him as much satisfaction as all the rest put together. And this was the gift: The Romans who were unhappily taken prisoners in the war with Hannibal had been sold about hither and thither, and were serving as slaves. In Greece there were as many as twelve hundred of them. The change in their lot made them pitiful objects always, but then even more than ever, naturally, when they fell in with sons, or brothers, or familiar friends, as the case might be, slaves with freemen and captives with victors. These men Titus would not take away from their owners, although he was distressed at their condition, but the Achaeans ransomed them all at five minas the man, collected them together, and made a present of them to Titus just as he was about to embark, so that he sailed for home with a glad heart; his noble deeds had brought him a noble recompense, and one befitting a great man who loved his fellow citizens. This appears to have furnished his triumph with its most glorious feature. For these men shaved their heads and wore felt caps, as it is customary for slaves to do when they are set free, and in this habit followed the triumphal car of Titus.
Nice call Alibegoa. Granted, Plutarch says nothing specifically about these slaves being taken from Cannae, but it's a reasonable inference.