Silsilat al-Ahadith al-Sahihah

Sayyiduna Abu Umamah (may Allah be pleased with him) narrates that the Prophet (peace and blessings of Allah be upon him) came from somewhere along with two slaves. He gifted one of them to Sayyiduna Ali (may Allah send blessings upon him) and said: "Do not beat him, for I have been forbidden from beating those who pray, and ever since we set out from there, I have seen him performing prayer." The other slave was given to Sayyiduna Abu Dharr (may Allah be pleased with him), and he said: "Treat him kindly." So he set him free. (One day) the Prophet (peace and blessings of Allah be upon him) asked him (about the slave): "How is he doing?" He replied: "You advised me to treat him kindly, so (acting upon your advice) I have set him free."