Sunan Ibn Majah, Chapter: The Chapters on Rulings

"My grandfather was Munqidh bin 'Amr. He was a man who had suffered a head wound and lost the power of speech, but that did not stop him from engaging in trade. He was always being cheated, so he went to the Prophet (saw) and told him about that. He said to him: 'When you buy something, say: "There should be no intention of cheating," and for every product you buy, you have the choice for three nights. If you are pleased with it, keep it, and if you are displeased then return it.'"