Sahih al-Bukhari 2414
Sahih al-Bukhari · Book 44: Khusoomaat · If somebody sells a thing for a weak-minded person and pays him the price, and advised him to utilize it in a proper way and not to spoil it, and he spoils it, he can stop him from doing so, for the Prophet had forbidden the wasting of the property. The Prophet ordered the person who said that he had always been cheated in buying, to say, at the time of the deal, "No cheating!" The Prophet did not take his money (of the person who sold his slave, because he had not proved foolish).
Narrated Ibn `Umar:
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ ، حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ مُسْلِمٍ ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ دِينَارٍ ، قَالَ سَمِعْتُ ابْنَ عُمَرَ ـ رضى الله عنهما ـ قَالَ كَانَ رَجُلٌ يُخْدَعُ فِي الْبَيْعِ فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم " إِذَا بَايَعْتَ فَقُلْ لاَ خِلاَبَةَ " . فَكَانَ يَقُولُهُ .
A man was often cheated in buying. The Prophet (ﷺ) said to him, "When you buy something, say (to the seller), No cheating." The man used to say so thenceforward .