Sahih al-Bukhari 2407
Sahih al-Bukhari · Book 43: Loans, Payment of Loans, Freezing of Property, Bankruptcy · Wasting money
Narrated Ibn `Umar:
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 .
A man came to the Prophet (ﷺ) and said, "I am often betrayed in bargaining." The Prophet (ﷺ) advised him, "When you buy something, say (to the seller), 'No deception." The man used to say so afterwards.