Musannaf Ibn Abi Shayba 36:422

Narrated to us Abd al-Rahim ibn Sulayman, from al-Hajjaj, from al-Hakam, from Miqsam, from Ibn Abbas: He said: Two young boys went out to the Prophet, may Allah bless him and grant him peace, on the day of Ta'if, so he emancipated them. One of them was Abu Bakrah, and they became his freedmen.