First Law of Braavos
The First Law of Braavos is that no man, woman, or child in Braavos will ever be a slave, thrall, or bondsman. This law was set down by the founders of Braavos, who were escaped slaves from Valyria. The law is engraved in stone on an arch spanning the Long Canal.
A Dance with Dragons
Arya Stark, blinded and going by the name "Blind Beth", learns of the Goodheart, a Lyseni pirate ship that has been commandeered by the Sealord of Braavos. The ship was carrying hundreds of women and children as slaves to Lys, wildlings from Hardhome, and had to put in at Braavos when it was damaged by a storm, despite the fact of slavery being forbidden there. In Pynto's tavern, Arya overhears Lyseni sailors off the Goodheart speaking of their situation, and the next day reports this news to the kindly man at the House of Black and White.