Musnad Ahmad 25:391

Nasr ibn Bab narrated to us, from al-Hajjaj, from Miqsam, from Ibn Abbas: That the Messenger of Allah, peace and blessings of Allah be upon him, emancipated on the day of Ta'if whoever came out to him from among the slaves. Nasr ibn Bab narrated to us, from al-Hajjaj, from al-Hakam, from Miqsam, from Ibn Abbas, that he said: The Messenger of Allah, peace and blessings of Allah be upon him, said on the day of Ta'if: "Whoever comes out to us from among the slaves, he is free." So, slaves came out, among them was Abu Bakrah, and the Messenger of Allah, peace and blessings of Allah be upon him, emancipated them.