Musannaf Ibn Abi Shayba 27:838

Muhammad ibn Abd Allah ibn al-Zubayr al-Asadi narrated to us, from Ibn Abi Dhi’b, from al-Zuhri, from Sa’id ibn al-Musayyib, that ‘Umar forbade an Arab man from marrying a slave woman. And he decreed regarding the Arabs who marry slave women, that their children are [to be redeemed] by a ransom of six young she-camels, for men and women alike. And the *Mawali* (non-Arab freedmen) are the same as that, if it is not known. Al-Zuhri said: The Arab and the *Mawla* are not equal in lineage.