Sayyiduna Qays bin Abi Gharazah (may Allah be pleased with him) says: We used to buy and sell in the markets and called ourselves brokers. He (peace and blessings be upon him) said: "O company of merchants, swearing is customary in these markets of yours, so mix it with charity, or with something of charity."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10413