Sahih al-Bukhari 2042
In-book no 2042حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا 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 ".
Narrated `Abdullah bin `Umar: A person came to the Prophet (ﷺ) and told him that he was always betrayed in purchasing. The Prophet (ﷺ) told him to say at the time of buying, "No cheating."