Musnad Ahmad 27:855

'Abd al-Rahman narrated to us, from Malik, from Nafi', from Ibn 'Umar, that the Prophet, peace and blessings of Allah be upon him, forbade meeting the goods [before they reach the market] until they are brought down to the markets. And he forbade *al-najash* (bidding up the price without the intention to buy). And he said: "Let not some of you sell over the sale of others."