Sahih al-Bukhari Book 34, Hadith 39
My father bought a slave who practiced the profession of cupping. (My father broke the slave's
instruments of cupping). I asked my father why he had done so. He replied, "The Prophet (ﷺ) forbade the
acceptance of the price of a dog or blood, and also forbade the profession of tattooing, getting tattooed
and receiving or giving Riba, (usury), and cursed the picture-makers."
The Prophet (ﷺ) said, "This night I dreamt that two men came and took me to a Holy land whence we proceeded on till we reached a river of blood, where a man was standing, and on its bank was standing another man with stones in his hands. The man in the middle of the river tried to come out, but the other threw a stone in his mouth and forced him to go back to his original place. So, whenever he tried to come out, the other man would throw a stone in his mouth and force him to go back to his former place. I asked, 'Who is this?' I was told, 'The person in the river was a Riba-eater."
I heard Allah's Messenger (ﷺ) saying, "The swearing (by the seller) may persuade the buyer to purchase the goods but that will be deprived of Allah's blessing."