Sunan an-Nasai Book 44, Hadith 4485

أَخْبَرَنَا يُوسُفُ بْنُ حَمَّادٍ، قَالَ حَدَّثَنَا عَبْدُ الأَعْلَى، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ رَجُلاً، كَانَ فِي عُقْدَتِهِ ضَعْفٌ كَانَ يُبَايِعُ وَأَنَّ أَهْلَهُ أَتَوُا النَّبِيَّ صلى الله عليه وسلم فَقَالُوا يَا نَبِيَّ اللَّهِ احْجُرْ عَلَيْهِ ‏.‏ فَدَعَاهُ نَبِيُّ اللَّهِ صلى الله عليه وسلم فَنَهَاهُ فَقَالَ يَا نَبِيَّ اللَّهِ إِنِّي لاَ أَصْبِرُ عَنِ الْبَيْعِ ‏.‏ قَالَ ‏ "‏ إِذَا بِعْتَ فَقُلْ لاَ خِلاَبَةَ ‏"‏ ‏.‏

It was narrated from Anas that: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)