Al-Adab Al-Mufrad #163

Abu Umama said, "The Prophet, may Allah bless him and grant him peace, came with two slaves and gave one of them to 'Ali and said, 'Do not beat him. I have forbidden beating the people of the prayer and I saw him praying before we came.' He gave Abu Dharr a slave and said, "I recommend that you treat him well,' so Abu Dharr set him free. He said, 'What have you done?' He replied, 'You commanded me to treat him well, so I set him free.'"