Sahih al-Bukhari 2415

Chapter: 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).

Jabir