It is narrated from Sayyiduna Amr bin Awf bin Milhah al-Muzani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There is no jalab (that is, bringing the animals away from their grazing place in order to collect zakah), no janab (that is, taking a fast horse tied alongside in a race), and no (unlawful) interference (in buying and selling); and no town-dweller should sell (as a broker) the goods of a desert-dweller."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14087