Sahih Adab al-Mufrad

It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that he said: The Prophet (peace and blessings of Allah be upon him) came and with him were two slaves. The Prophet (peace and blessings of Allah be upon him) gave one of them to Sayyiduna Ali (may Allah's blessings be upon him) and said: "Do not beat him, I have been forbidden from beating those who pray, and since we have come, I have seen him performing prayer." And he gave the other slave to Sayyiduna Abu Dharr (may Allah be pleased with him) and said: "Take my advice to treat him well." Sayyiduna Abu Dharr (may Allah be pleased with him) set him free. The Prophet (peace and blessings of Allah be upon him) asked: "What did you do with him?" Sayyiduna Abu Dharr (may Allah be pleased with him) said: You commanded me to treat him well, so I set him free.