Musannaf Ibn Abi Shayba 13:2271

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from al-Taymi, from al-Hasan ibn Muslim: He said: Umar ibn Abd al-Aziz wrote to the people of Medina and to the people of Mecca—or one of the two—and he forbade them from boycotting the *mukātabīn* (slaves under a contract of manumission). He said: I do not see any harm in this.