Musnad Ahmad 27:956

Abu Salamah narrated to us, Sulayman ibn Bilal informed us, from Abdullah ibn Dinar, from Ibn Umar, who said: A man was mentioned to the Prophet, peace and blessings of Allah be upon him, who would be deceived in buying and selling. He said to him: "When you engage in a transaction, say: 'No deception (la khilabah).'" So he would say when he engaged in a transaction: "No deception." And there was a stutter in his tongue.