Musnad Ahmad 27:1511

Al-Fadl ibn Dukayn narrated to us, Sufyan narrated to us, from Abdullah ibn Dinar, I heard Ibn Umar saying: A man said to the Prophet, may Allah bless him and grant him peace: "I am deceived in buying and selling." He said: "When you trade, say: 'No deception (la khilaba).'" So the man used to say it.