First Law of Braavos
From A Wiki of Ice and Fire
The First Law of Braavos is that no man, woman or child will ever be a slave, thrall, or bondsman. This law was set down by the founders of Braavos, escaped slaves from Valyria themselves. It is upheld to this day, and in A Dance with Dragons Arya Stark (at the time Blind Beth) learns of a ship that is carrying wildlings from Hardhome that has been commandeered by the Sealord of Braavos.