Sahih al-Bukhari, Book 34, Hadith 70 (In-Book) ⓘ
Narrated `Abdullah bin `Umar: A person came to the Prophet and told him that he was always betrayed in purchasing. The Prophet told him to say at the time of buying, "No cheating."
حَدَّثَنَا 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 " .
also referenced under: Hadith 2117 (DarusSalam) & Volume 3, Book 34, Hadith 328 (USC-MSA)