Sahih al-Bukhari, Book 43, Hadith 22 (In-Book) ⓘ
Narrated Ibn `Umar: A man came to the Prophet and said, "I am often betrayed in bargaining." The Prophet advised him, "When you buy something, say (to the seller), 'No deception." The man used to say so afterwards.
حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا سُفْيَانُ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، سَمِعْتُ ابْنَ عُمَرَ ـ رضى الله عنهما ـ قَالَ قَالَ رَجُلٌ لِلنَّبِيِّ صلى الله عليه وسلم إِنِّي أُخْدَعُ فِي الْبُيُوعِ. فَقَالَ " إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ " . فَكَانَ الرَّجُلُ يَقُولُهُ.
also referenced under: Hadith 2407 (DarusSalam) & Volume 3, Book 41, Hadith 590 (USC-MSA)