Musnad Ahmad 31:685

Hassan narrated to us, Hammad ibn Salamah narrated to us, from Hammad, from Ibrahim, from Abu Sa’id al-Khudri: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade hiring a worker until his wage is clarified to him. And [he forbade] the throwing of a stone, the touching [of goods], and the bidding up of prices (al-najash)."