A man mentioned to the Messenger of Allah ﷺ that he was being deceived in transactions.The Messenger of Allah ﷺ said, "When you make a deal, say, 'No deception.'"So whenever that man made a transaction, he would say, "No deception."
أن رجلا ذكر لرسول الله ﷺ أنه يخدع في البيوع، فقال رسول الله ﷺ: إذا بايعت فقل: لا خلابة قال: فكان الرجل إذا بايع يقول: لا خلابة.
Malik narrated it in the Book of Sales (98), from Abdullah bin Dinar, from Abdullah bin Umar, and he mentioned it. Bukhari narrated it in the Book of Sales (2117) via Malik, with the same chain. Muslim narrated it in the Book of Sales (1533) through another route from Abdullah bin Dinar, with the same chain.