Narrated Ibn 'Umar (RA):

A man told Allah's Messenger (ﷺ) that he was being deceived (cheated) in business transactions, and he replied, "When you make a purchase say, 'Deceiving (cheating) is not allowed (i.e. he has the choice to return the goods if he wills).'" [Agreed upon].

وَعَنِ اِبْنِ عُمَرَ ‏-رَضِيَ اَللَّهُ عَنْهُمَا‏- قَالَ: ذَكَرَ رَجُلٌ لِلنَّبِيِّ ‏- صلى الله عليه وسلم ‏-أَنَّهُ يُخْدَعُ فِي اَلْبُيُوعِ فَقَالَ: { إِذَا بَايَعْتَ فَقُلْ: لَا خَلَابَةَ } مُتَّفَقٌ عَلَيْهِ [1]‏ .‏