Musnad Ahmad 27:1066

'Abd al-Razzaq narrated to us, Sufyan informed us, from 'Abdullah ibn Dinar, from Ibn 'Umar, who said: A man asked the Prophet, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, I am a man who is deceived in buying and selling." The Prophet, peace and blessings of Allah be upon him, said: "Whenever you engage in a transaction, say: 'There is no deception.'"