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.)