Musnad Ahmad 27:1674

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Nafi' narrated to me, from Ibn 'Umar, who said: There was a man from the Ansar who would constantly be cheated in sales, and there was a stutter in his tongue. He complained to the Messenger of Allah (peace and blessings of Allah be upon him) about what he encountered of being cheated. The Messenger of Allah (peace and blessings of Allah be upon him) said to him: "When you conduct a sale, say: 'No deception.'" Ibn 'Umar used to say: "By Allah, it is as if I can hear him conducting a sale and saying: 'No deception,' stuttering with his tongue."