بُلُوغُ الْمَرَامِ
Bulugh al-Maram 828
Business Transactions
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] .
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].
Sunnah.com reference: Book 7, Hadith 61