Sahih Al Bukhari Hadith Number 2414

Narrated Ibn `Umar: A man was often cheated in buying. The Prophet (peace be upon him) said to him, “When you buy something, say (to the seller), No cheating.” The man used to say so thenceforward .

44  Khusoomaat
Hadith number 2414
Volume no 3
Chapter: 3 (If somebody sells a thing for a weak-minded person)