Musannaf Ibn Abi Shayba 9:1026

Waki’ narrated to us, from Ibn Abi Dhi’b, from Shu’bah, the freed slave of Ibn Abbas, from Ibn Abbas, who said: There were prostitutes in the pre-Islamic period (Jahiliyyah) who would place flags upon their doors like the flags of the veterinarians. People would come to them, and they were known by that.