Sahih Bukhari Volume 003, Book 041, Hadith Number 597.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 41. Loans, Payment of Loans, Freezing of Property, Bankruptcy.
Narated By Ibn 'Umar : A man was often cheated in buying. The Prophet said to him, "When you buy something, say (to the seller), No cheating." The man used to say so thenceforward.