Sahih al-Bukhari 6964
Sahih al-Bukhari · Book 90: Tricks · Cheating in bargains
Narrated `Abdullah bin `Umar:
حَدَّثَنَا إِسْمَاعِيلُ ، حَدَّثَنَا مَالِكٌ ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَجُلاً، ذَكَرَ لِلنَّبِيِّ صلى الله عليه وسلم أَنَّهُ يُخْدَعُ فِي الْبُيُوعِ فَقَالَ " إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ " .
A man mentioned to the Prophet (ﷺ) that he had always been cheated in bargains. The Prophet (ﷺ) said, "Whenever you do bargain, say, 'No cheating.'"