It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that a man mentioned to the Messenger of Allah (peace and blessings be upon him) that he is deceived in transactions, so the Messenger of Allah (peace and blessings be upon him) said to him: "When you engage in a transaction, say: 'Do not deceive me.'" Then, whenever he engaged in a transaction, he would say: "Do not deceive me."
Hadith Referenceالسنن المأثورة / باب في البيوع / 261
Hadith Takhrijصحیح بخاری، البيوع، باب ما يكره من الخداع في البيع، رقم : 2117، صحیح مسلم، البيوع، باب من يخدع في البيع، رقم : 1533