Sahih al-Bukhari, Volume 3, Book 41, Hadith 597 (USC-MSA) ⓘ
Narrated Ibn `Umar: A man was often cheated in buying. The Prophet said to him, "When you buy something, say (to the seller), No cheating." The man used to say so thenceforward.
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ مُسْلِمٍ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ دِينَارٍ، قَالَ سَمِعْتُ ابْنَ عُمَرَ ـ رضى الله عنهما ـ قَالَ كَانَ رَجُلٌ يُخْدَعُ فِي الْبَيْعِ فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم " إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ " . فَكَانَ يَقُولُهُ.
also referenced under: Book 44, Hadith 5 (In-Book) & Hadith 2414 (DarusSalam)