Abdullah b. Dinar narrated that he heard Ibn 'Umar (Allaah be pleased with them) saying: A man mentioned to the Messenger of Allaah (sallAllaahu alayhi wa sallam) that he was deceived in a business transaction, whereupon Allaah's Messenger (sallAllaahu alayhi wa sallam) said: When you enter into a transaction, say: There should be no attempt to deceive.
This hadith has been narrated on the authority of 'Abdullah b. Dinar with the same chain of transmitters but these words are not found in it." When he buys he should say: There should be no attempt to deceive."