Musnad Ahmad 30:1132

He said: "He forbade two types of sales and two types of dress: that one of you should sit in a single garment with nothing of it covering his private parts, and that one should wrap himself in his lower garment while praying, unless he crosses its ends over his shoulders. And he forbade *al-lams* (touching) and *al-najash* (bidding up the price without the intention to buy)."