Musnad Ahmad 30:3385

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not meet the riders (bringing goods to the market) for the purpose of trade, and let no resident sell on behalf of a bedouin. Do not hate one another, do not envy one another, and do not outbid one another (to deceive). And be, O servants of Allah, brothers."