وعنه قال: ذكر رجل لرسول الله صلى الله عليه وسلم أنه يُخدع في البيوع فقال رسول الله صلى الله عليه وسلم: «من بايعت؟ فقل: لا خلابة»
A man mentioned to the Messenger of Allah (sallallahu alayhi wa sallam) that he was often deceived in dealings. The Messenger of Allah (sallallahu alayhi wa sallam) said to him, “When you enter into a transaction you should say: ‘There should be no deception.’”.
Riyad as-Salihin 1582, Book 18, The Book of the Prohibited actions, Hadith 72. 1 min read