A man mentioned to the Messenger of Allah ﷺ that he was being deceived in business transactions. The Messenger of Allah ﷺ said to him, "When you make a deal, say, 'No deception.'" So whenever the man made a deal, he would say, "No deception."
حدثنا عبد الله بن مسلمة عن مالك عن عبد الله بن دينار
أن رجلا ذكر لرسول الله ﷺ أنه يخدع في البيع فقال له رسول الله ﷺ إذا بايعت فقل لا خلابة فكان الرجل إذا بايع يقول لا خلابة
Bukhari narrated it in the Book of Business Transactions, chapter "What is Disliked of Deception in Selling," hadith no. 2117, from the hadith of Malik. Muslim narrated it in the Book of Business Transactions, chapter "One Who is Deceived in Selling," hadith no. 1533, from the hadith of Abdullah bin Dinar, through this same chain. It is also found in Muwatta (Yahya's recension) 2/685.