Musannaf Ibn Abi Shayba 28:1069

Yazid ibn Harun narrated to us, from al-Hajjaj, from al-Hakam, from Miqsam, from Ibn Abbas, that the Prophet (peace and blessings of Allah be upon him) used to emancipate those who came to him from the slaves before their masters if they embraced Islam. And he emancipated two men on the day of Ta'if.