Musnad Abu Ya'la al-Mawsili 154:166

Mus'ab and Suwayd ibn Sa'id narrated to us. Mus'ab said: "He narrated to me," and Suwayd said: "From Nafi', from Ibn 'Umar, that the Prophet (peace and blessings of Allah be upon him) forbade *al-najash* (bidding up the price of an item without the intention of buying it)."