Sahih Bukhari
Manumission of Slaves
Volume: 003 / Book Number: 046 / Hadith Number: 709
Narrated Qais:
When Abu Huraira accompanied by his slave came intending to embrace Islam, they lost each other on the way. (When the slave showed up) Abu Huraira said (to the Prophet), "I make you witness that the slave is free for Allah's Cause."