Musnad Ahmad 27:1991

Hammad ibn Khalid narrated to us, Malik narrated to us, from Nafi’, from Ibn ‘Umar, that the Prophet (peace and blessings of Allah be upon him) said: "Do not meet the riders (to buy their goods before they reach the market)." And he forbade *an-najash* (bidding up the price of an item without the intention to buy it).