Musnad Ahmad 30:3665

Yahya ibn Adam narrated to us, Sufyan narrated to us, from Salih ibn Nabhan, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A resident shall not sell for a bedouin. Do not turn your backs on one another, do not outbid one another to deceive, and be, O servants of Allah, brothers."