Sahih Muslim | Book 10 | Chapter 12

Narrated by Abdullah b. Dinar
Abdullah b. Dinar narrated that he heard Ibn 'Umar (Allah be pleased with them) saying: A man mentioned to the Messenger of Allah (may peace be upon him) that he was deceived in a business transaction, whereupon Allah's Messenger (may peace be upon him) said: When you enter into a transaction, say: There should be no attempt to deceive.

Narrated by 'Abdullah b. Dinar
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.