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."  

أَنَّ رَجُلاً ذَكَرَ لِلنَّبِيِّ ﷺ أَنَّهُ يُخْدَعُ فِي الْبُيُوعِ فَقَالَ إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ