Musnad Ahmad 285:5

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Habib ibn Abi Thabit, from Abu Wa'il, from Qays ibn Abi Gharzah, who said: We used to sell slaves in the market, and we were called brokers. The Messenger of Allah (may Allah bless him and grant him peace) named us with a better name than what we had named ourselves. He said: "O assembly of merchants, this selling is attended by idle talk and oaths, so mix it with charity."