Musannaf Ibn Abi Shayba 28:102

Ibn Fudayl narrated to us, from Sadaqah, from Rabah ibn al-Harith, who said: Umar used to judge regarding what the Arabs had enslaved of one another before Islam and before the Prophet, may Allah bless him and grant him peace, was sent: That whoever recognized anyone from his household as a slave belonging to one of the tribes of the Arabs, then the ransom for the male slave is two male slaves, and for the female slave is two female slaves.