Maliks Muwatta Book 031, Hadith Number 099.

Section : Business Transactions in General.

Yahya related to me from Malik from Abdullah ibn Dinar from Abdullah ibn Umar that a man mentioned to the Messenger of Allah, may Allah bless him and grant him peace, that he was always being cheated in business transactions. The Messenger of Allah, may Allah bless him and grant him peace, said, "When you enter a transaction, say, 'No trickery.' So whenever that man entered a transaction, he would say, 'No trickery.'"