a man told the Messenger of Allah that he was always being cheated. The Messenger of Allah said to him: "When you make a deal, say: There is no intention of cheating" So, whenever the man engages in a deal he says, 'There is no intention of cheating." "(Sahih )  

أَنَّ رَجُلاً ذَكَرَ لِرَسُولِ اللَّهِ ﷺ أَنَّهُ يُخْدَعُ فِي الْبَيْعِ فَقَالَ لَهُ رَسُولُ اللَّهِ ﷺ إِذَا بِعْتَ فَقُلْ لاَ خِلاَبَةَ فَكَانَ الرَّجُلُ إِذَا بَاعَ يَقُولُ لاَ خِلاَبَةَ  

there was a man whose mental faculties were lacking, and he used to buy and sell. His family came to the Prophet and said: "O Prophet of Allah stop him." So the Prophet of Allah called him, and told him not to do that. He said: "O Prophet of Allah, I cannot bear to be away from business," He said "If you engaged in a deal then say: 'There is no intention to deceive. "' (Sahih )  

أَنَّ رَجُلاً كَانَ فِي عُقْدَتِهِ ضَعْفٌ كَانَ يُبَايِعُ وَأَنَّ أَهْلَهُ أَتَوُا النَّبِيَّ ﷺ فَقَالُوا يَا نَبِيَّ اللَّهِ احْجُرْ عَلَيْهِ فَدَعَاهُ نَبِيُّ اللَّهِ ﷺ فَنَهَاهُ فَقَالَ يَا نَبِيَّ اللَّهِ إِنِّي لاَ أَصْبِرُ عَنِ الْبَيْعِ قَالَ إِذَا بِعْتَ فَقُلْ لاَ خِلاَبَةَ