Sahih Adab al-Mufrad

Yazid bin Abdullah bin Qasit reports that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) sent his slave to the market with gold or silver. The slave sold it in exchange but delayed in taking the exchange. When the slave returned, Abdullah bin Umar beat him very severely. He said: Go and bring back the capital that was mine, and do not bring anything in exchange for it. (That is, do not bring silver in exchange for gold, nor gold in exchange for silver.)

It is narrated from Sayyiduna Abu Mas’ud (may Allah be pleased with him) who says: I was beating my slave when I heard a voice from behind me, “O Abu Mas’ud! Know that Allah has more power over you than you have over him.” When I turned around, it was the Messenger of Allah (peace and blessings be upon him). I said, “O Messenger of Allah, I have freed him for the sake of Allah.” The Prophet (peace and blessings be upon him) said, “Beware! If you had not done so, the Fire would have engulfed you or the Fire would have touched you.”