Sahih Bukhari
Loans, Payment of Loans, Freezing of Property, Bankruptcy
Volume: 003 / Book Number: 041 / Hadith Number: 590
Narrated Ibn 'Umar:
A man came to the Prophet and said, "I am often betrayed in bargaining." The Prophet advised him, "When you buy something, say (to the seller), 'No deception." The man used to say so afterwards.