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Narrated Ibn 'Umar: A man told the Messenger of Allah (ﷺ) that he was being deceived in business transactions. The Messenger of Allah (ﷺ) then said: When you make a bargain, say: There is no attempt to deceive. So when the man made a bargain, he said: There is no attempt to deceive.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَجُلاً، ذَكَرَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ يُخْدَعُ فِي الْبَيْعِ فَقَالَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ " . فَكَانَ الرَّجُلُ إِذَا بَايَعَ يَقُولُ لاَ خِلاَبَةَ .
sunnah.com