Musannaf Ibn Abi Shaybah 39729 — Reports on Taif

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that two slaves came out to the Prophet (peace and blessings be upon him) on the day of al-Ta'if, and he (peace and blessings be upon him) set them both free. One of them was Sayyiduna Abu Bakrah (may Allah be pleased with him). So these two were his (peace and blessings be upon him) mawali (freed slaves).